@string{brics =	"{BRICS}"}
@string{daimi =	"Department of Computer Science, University of Aarhus"}
@string{iesd  =	"Department of Computer Science, Institute
of Electronic Systems, Aalborg University"}
@string{rs    =	"Research Series"}
@string{ns    =	"Notes Series"}
@string{ls    =	"Lecture Series"}
@string{ds    =	"Dissertation Series"}

@techreport{BRICS-RS-99-57,
author = 	 "Mosses, Peter D.",
title = 	 "A Modular {SOS} for {ML} Concurrency
Primitives",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-57",
month = 	 dec,
note = 	 "22~pp",
abstract = 	 "Modularity is an important pragmatic aspect of
semantic descriptions. In denotational
semantics, the issue of modularity has received
much attention, and appropriate abstractions
have been introduced, so that definitions of
semantic functions may be independent of the
details of how computations are modelled. In
structural operational semantics (SOS),
however, this issue has largely been neglected,
and SOS descriptions of programming languages
typically exhibit rather poor modularity---for
instance, extending the described language may
entail a complete reformulation of the
description of the original constructs.\bibpar
A proposal has recently been made for a modular
approach to SOS, called MSOS\@. The basic
definitions of the Modular SOS framework are
recalled here, but the reader is referred to
other papers for a full introduction. This
paper focusses on illustrating the
applicability of Modular SOS, by using it to
give a description of a sublanguage of
Concurrent ML (CML)\@; the transition rules for
the purely functional constructs do not have to
be reformulated at all when adding references
and/or processes. The paper concludes by
comparing the MSOS description with previous
operational descriptions of similar
languages.\bibpar
The reader is assumed to be familiar with
conventional SOS, with the concepts of
functional programming languages such as
Standard ML, and with fundamental notions of
concurrency",
}

@techreport{BRICS-RS-99-56,
author = 	 "Mosses, Peter D.",
title = 	 "A Modular {SOS} for Action Notation",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-56",
month = 	 dec,
note = 	 "39~pp. Full version of paper appearing in " #
ns993 # ", pages 131--142",
abstract = 	 "Modularity is an important pragmatic aspect of
semantic descriptions: good modularity is
needed to allow the reuse of existing
descriptions when extending or changing the
described language. In denotational semantics,
the issue of modularity has received much
attention, and appropriate abstractions have
been introduced, so that definitions of
semantic functions may be independent of the
details of how computations are modelled. In
structural operational semantics (SOS),
however, this issue has largely been neglected,
and SOS descriptions of programming languages
typically exhibit rather poor modularity; the
original SOS given for Action Notation (the
notation for the semantic entities used in
action semantics) suffered from the same
problem.\bibpar
This paper recalls a recent proposal, called
MSOS, for obtaining a high degree of modularity
in SOS, and presents an MSOS description of
Action Notation. Due to its modularity, the
MSOS description pin-points some complications
in the design of Action Notation, and should
facilitate the design of an improved version of
the notation. It also provides a major example
of the applicability of the MSOS
framework.\bibpar
The reader is assumed to be familiar with
conventional SOS and with the basic concepts
and constructs of Action Notation. The
description of Action Notation is formulated
almost entirely in {\sc Casl}, the common
algebraic specification language",
}

@techreport{BRICS-RS-99-55,
author = 	 "Mosses, Peter D.",
title = 	 "Logical Specification of Operational
Semantics",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-55",
month = 	 dec,
note = 	 "18~pp. Invited paper. Appears in " # lncs1683
# ", pages 32--49",
abstract = 	 "Various logic-based frameworks have been
proposed for specifying the operational
semantics of programming languages and
concurrent systems, including inference systems
in the styles advocated by Plotkin and by Kahn,
Horn logic, equational specifications,
reduction systems for evaluation contexts,
rewriting logic, and tile logic.\bibpar
We consider the relationship between these
frameworks, and assess their respective merits
and drawbacks---especially with regard to the
modularity of specifications, which is a
crucial feature for scaling up to practical
applications. We also report on recent work
towards the use of the Maude system (which
provides an efficient implementation of
rewriting logic) as a meta-tool for operational
semantics",
}

@techreport{BRICS-RS-99-54,
author = 	 "Mosses, Peter D.",
title = 	 "Foundations of Modular {SOS}",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-54",
month = 	 dec,
note = 	 "17~pp. Full version of paper appearing in " #
lncs1672 # ", pages 70--80",
abstract = 	 "A novel form of labelled transition system is
proposed, where the labels are the arrows of a
category, and adjacent labels in computations
are required to be composable. Such transition
systems provide the foundations for modular SOS
descriptions of programming languages.\bibpar
Three fundamental ways of transforming label
are provided, and it is shown that their
applications preserve computations in modular
SOS. The approach is illustrated with fragments
taken from a modular SOS for ML concurrency
primitives.",
}

@techreport{BRICS-RS-99-53,
author = 	 "Iversen, Torsten K. and Kristoffersen,
K{\aa}re J. and Larsen, Kim G. and Laursen,
Morten and Madsen, Rune G. and Mortensen,
Steffen K. and Pettersson, Paul and Thomasen,
Chris B.",
title = 	 "Model-Checking Real-Time Control Programs ---
Verifying {LEGO} Mindstorms Systems Using
{UPPAAL}",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-53",
month = 	 dec,
note = 	 "9~pp. Appears in " # ieeeecrts00 # ", pages
147--155",
abstract = 	 "In this paper, we present a method for
automatic verification of real-time control
programs running on LEGO\textsuperscript
{\textcircled{\tiny R}} RCX\textsuperscript
{\textsf{T$\!$M}} bricks using the verification
tool {\sc Uppaal}. The control programs,
consisting of a number of tasks running
concurrently, are automatically translated into
the timed automata model of {\sc Uppaal}. The
fixed scheduling algorithm used by the
LEGO\textsuperscript{\textcircled{\tiny R}}
RCX\textsuperscript{\textsf{T$\!$M}} processor
is modeled in {\sc Uppaal}, and supply of
similar (sufficient) timed automata models for
the environment allows analysis of the overall
real-time system using the tools of {\sc
Uppaal}. To illustrate our techniques we have
constructed, modeled and verified a machine for
sorting LEGO\textsuperscript{\textcircled{\tiny
R}} bricks by color",
}
@techreport{BRICS-RS-99-52,
author = 	 "Henriksen, Jesper G. and Mukund, Madhavan and
Narayan Kumar, K. and Thiagarajan, P. S.",
title = 	 "Towards a Theory of Regular {MSC} Languages",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-52",
month = 	 dec,
note = 	 "43~pp",
abstract = 	 "Message Sequence Charts (MSCs) are an
attractive visual formalism widely used to
capture system requirements during the early
design stages in domains such as
telecommunication software. It is fruitful to
have mechanisms for specifying and reasoning
about collections of MSCs so that errors can be
detected even at the requirements level. We
propose, accordingly, a notion of {\em
regularity} for collections of MSCs and explore
its basic properties. In particular, we provide
an automata-theoretic characterization of
regular MSC languages in terms of finite-state
distributed automata called bounded
message-passing automata. These automata
consist of a set of sequential processes that
communicate with each other by sending and
receiving messages over bounded FIFO channels.
We also provide a logical characterization in
terms of a natural monadic second-order logic
interpreted over MSCs.\bibpar
A commonly used technique to generate a
collection of MSCs is to use a Message Sequence
Graph (MSG). We show that the class of
languages arising from the so-called locally synchronized
MSGs constitute a proper subclass of the
languages which are regular in our sense. In
fact, we characterize the locally synchronized MSG languages
as the subclass of regular MSC languages that
are {\em finitely generated}",
}

@techreport{BRICS-RS-99-51,
author =	 "Danvy, Olivier",
title =	 "Formalizing Implementation Strategies for
First-Class Continuations",
institution =	 brics,
year =	 1999,
type =	 rs,
number =	 "RS-99-51",
month =	 dec,
note =	 "16~pp. Appears in " # lncs1782 # "pp. 88--103",
abstract =	 "We present the first formalization of implementation
strategies for first-class continuations. The
formalization hinges on abstract machines for
continuation-passing style (CPS) programs with a
special treatment for the current continuation,
accounting for the essence of first-class
continuations. These abstract machines are proven
equivalent to a standard, substitution-based
abstract machine. The proof techniques work
uniformly for various representations of
continuations. As a byproduct, we also present a
formal proof of the two folklore theorems that one
continuation identifier is enough for second-class
continuations and that second-class continuations
are stackable.\bibpar A large body of work exists on
implementing continuations, but it is predominantly
empirical and implementation-oriented. In contrast,
our formalization abstracts the essence of
first-class continuations and provides a uniform
setting for specifying and formalizing their
representation",
}
@techreport{BRICS-RS-99-50,
author =	 "Brodal, Gerth St{\o}lting and Venkatesh, Srinivasan",
title =	 "Improved Bounds for Dictionary Look-up with One
Error",
institution =	 brics,
year =	 1999,
type =	 rs,
number =	 "RS-99-50",
month =	 dec,
note =	 "5~pp. Appears in {\em Information Processing
Letters} 75(1--2):57--59, 2000",
keywords =	 "Data Structures, Dictionaries, Hashing, Hamming
Distance",
abstract =	 "Given a dictionary $S$ of $n$ binary strings each of
length $m$, we consider the problem of designing a
data structure for $S$ that supports
$d$-\emph{queries}; given a binary query string $q$
of length~$m$, a $d$-query reports if there exists a
string in $S$ within Hamming distance~$d$ of~$q$. We
construct a data structure for the case $d=1$, that
requires space $O(n\log m)$ and has query time
$O(1)$ in a cell~probe model with word size
$m$. This generalizes and improves the previous
bounds of Yao and Yao for the problem in the
bit~probe model",
}

@techreport{BRICS-RS-99-49,
author = 	 "Ageev, Alexander A. and Sviridenko, Maxim I.",
title = 	 "An Approximation Algorithm for Hypergraph Max
$k$-Cut with Given Sizes of Parts",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-49",
month = 	 dec,
note = 	 "12~pp. Appears in " # lncs1879 # ", pages
32--49",
abstract = 	 "In this paper we demonstrate that the pipage
rounding technique can be applied to the
Hypergraph Max $k$-Cut problem with given sizes
of parts. We present a polynomial time
approximation algorithm and prove that its
performance guarantee is tight",
}
@techreport{BRICS-RS-99-48,
author = 	 "Pagh, Rasmus",
title = 	 "Faster Deterministic Dictionaries",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-48",
month = 	 dec,
note = 	 "14~pp. Appears in " # acmsoda00 # ", pages
487--493. Journal version in {\em Journal of
Algorithms}, 41(1):69--85, 2001, with the title
{\em Deterministic Dictionaries}",
abstract = 	 "We consider static dictionaries over the
universe $U=\{0,1\}^w$ on a unit-cost RAM with
word size $w$. Construction of a static
dictionary with linear space consumption and
constant lookup time can be done in linear
expected time by a randomized algorithm. In
contrast, the best previous deterministic
algorithm for constructing such a dictionary
with $n$ elements runs in time $O(n^{1+\epsilon })$ for $\epsilon>0$. This paper narrows the
gap between deterministic and randomized
algorithms exponentially, from the factor of
$n^\epsilon$ to an $O(\log n)$ factor. The
algorithm is weakly non-uniform, i.e. requires
certain precomputed constants dependent on $w$.
A by-product of the result is a lookup time vs
dictionaries, which is optimal for a realistic
class of deterministic hashing schemes.",
}
@techreport{BRICS-RS-99-47,
author = 	 "Miltersen, Peter Bro and Variyam,
Vinodchandran N.",
title = 	 "Derandomizing {A}rthur-{M}erlin Games using
Hitting Sets",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-47",
month = 	 dec,
note = 	 "21~pp. Appears in " # ieeefocs99 # ", pages
71--80",
abstract = 	 "We prove that {\rm\bf AM} (and hence Graph
Nonisomorphism) is in {\rm\bf NP} if for some
$\epsilon>0$, some language in {\rm\bf NE}
$\cap$ {\rm\bf coNE} requires nondeterministic
circuits of size $2^{\epsilon n}$. This
improves recent results of Arvind and
K{\"o}bler and of Klivans and Van Melkebeek who
proved the same conclusion, but under stronger
hardness assumptions, namely, either the
existence of a language in {\rm\bf NE} $\cap$
{\rm\bf coNE} which cannot be {\em
approximated} by nondeterministic circuits of
size less than $2^{\epsilon n}$ or the
existence of a language in {\rm\bf NE} $\cap$
{\rm\bf coNE} which requires {\em oracle
circuits} of size $2^{\epsilon n}$ with oracle
gates for {\rm SAT} (satisfiability).\bibpar

The previous results on derandomizing {\rm\bf
AM} were based on pseudorandom generators. In
contrast, our approach is based on a
strengthening of Andreev, Clementi and Rolim's
hitting set approach to derandomization. As a
spin-off, we show that this approach is strong
enough to give an easy (if the existence of
explicit dispersers can be assumed known) proof
of the following implication: For some
$\epsilon>0$, if there is a language in {\bf E}
which requires nondeterministic circuits of
size $2^{\epsilon n}$, then {\rm\bf P}={\rm\bf
BPP}. This differs from Impagliazzo and
Wigderson's theorem only'' by replacing
deterministic circuits with nondeterministic
ones",
}

@techreport{BRICS-RS-99-46,
author = 	 "Miltersen, Peter Bro and Variyam,
Vinodchandran N. and Watanabe, Osamu",
title = 	 "Super-Polynomial Versus Half-Exponential
Circuit Size in the Exponential Hierarchy",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-46",
month = 	 dec,
note = 	 "14~pp. Appears in " # lncs1627 # ", pages
210--220",
abstract = 	 "Lower bounds on circuit size were previously
established for functions in $\Sigma_2^p$,
${\mbox{\rm ZPP}}^{\mbox{\rm\scriptsize NP}}$,
${{\Sigma^{\mbox{\rm\small exp}}_2}}$, ${{\mbox {\rm ZPEXP}}^{\mbox{\rm\scriptsize NP}}}$ and
${{\mbox{\rm MA}}_{\mbox{\rm\small exp}}}$. We
investigate the general question: Given a time
bound $f(n)$. What is the best circuit size
lower bound that can be shown for the classes
${\mbox{{\rm MA-TIME}}}[f]$, ${\mbox{\rm ZP-TIME}}^ {\mbox{\rm\scriptsize NP}}[f], \ldots$ using the techniques currently known?
For the classes ${{\mbox{\rm MA}}_{\mbox{\rm \small exp}}}$, ${{\mbox{\rm ZPEXP}}^{\mbox{\rm \scriptsize NP}}}$ and ${{\Sigma^{\mbox{\rm \small exp}}_2}}$, the answer we get is
half-exponential''. Informally, a function
$f$ is said to be half-exponential if $f$
composed with itself is exponential",
}

@techreport{BRICS-RS-99-45,
author = 	 "Amtoft, Torben",
title = 	 "Partial Evaluation for Constraint-Based
Program Analyses",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-45",
month = 	 dec,
note = 	 "13~pp",
abstract = 	 "We report on a case study in the application
of partial evaluation, initiated by the desire
to speed up a constraint-based algorithm for
control-flow analysis. We designed and
implemented a dedicated partial evaluator, able
to specialize the analysis wrt.\ a given
constraint graph and thus remove the
interpretive overhead, and measured it with
Feeley's Scheme benchmarks. Even though the
gain turned out to be pretty limited, our
investigation yielded valuable feed back in
that it provided a better understanding of the
analysis, leading us to (re)invent an
incremental version. We believe this phenomenon
to be a quite frequent spin-off from using
partial evaluation, since the removal of
interpretive overhead will make the flow of
control more explicit and hence pinpoint the
sources of inefficiency. Finally, we observed
that partial evaluation in our case yields such
regular, low-level specialized programs that it
begs for runtime code generation",
}
@techreport{BRICS-RS-99-44,
author = 	 "Nestmann, Uwe and H{\"u}ttel, Hans and Kleist,
Josva and Merro, Massimo",
title = 	 "Aliasing Models for Mobile Objects",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-44",
month = 	 dec,
note = 	 "ii+46~pp. Appears in {\em Information and
Computation} 172:1--31, 2002. An extended
abstract of this revision, entitled {\em
Aliasing Models for Object Migration}, appeared
as Distinguished Paper in " # lncs1685 # ",
pages 1353--1368, which in turn is a revised
part of another paper called {\em Migration =
Cloning ; Aliasing} that appeared in " # fool6
# " and as such supersedes the corresponding
part of the earlier BRICS report RS-98-33",
abstract = 	 "In Obliq, a lexically scoped, distributed,
object-oriented programming language, object
migration was suggested as the creation of a
copy of an object's state at the target site,
followed by turning the object itself into an
alias, also called {\em surrogate}, for the
remote copy. We consider the creation of object
surrogates as an abstraction of the
above-mentioned style of migration. We
introduce {\O}jeblik, a typed distribution-free
subset of Obliq, and provide four different
configuration-style semantics, which only
differ in the respective {\em aliasing model}.
We show that two of the semantics, one of which
matches Obliq's implementation, render
migration unsafe, while our new proposal allows
for safe migration at least for a large class
of program contexts. In addition, we propose a
type system that allows a programmer to
statically guarantee that programs belong to
that class. Our work suggests a straightforward
repair of Obliq's aliasing model",
}
@techreport{BRICS-RS-99-43,
author = 	 "Nestmann, Uwe",
title = 	 "What is a Good' Encoding of Guarded Choice?",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-43",
month = 	 dec,
note = 	 "ii+34~pp. Appears in {\em Information and
Computation}, 156:287--319, 2000. This revised
report supersedes the earlier BRICS report
RS-97-45",
abstract = 	 "We study two encodings of the {\em
asynchronous $\pi$-calculus} with {\em
input-guarded choice} into its choice-free
fragment. One encoding is divergence-free, but
refines the atomic commitment of choice into
atomicity, but introduces divergence. The
divergent encoding is fully abstract with
respect to weak bisimulation, but the more
natural divergence-free encoding is not.
Instead, we show that it is fully abstract with
respect to {\it coupled simulation}, a slightly
coarser---but still coinductively
defined---equivalence that does not enforce
bisimilarity of internal branching decisions.
The correctness proofs for the two choice
encodings introduce a novel proof technique
exploiting the properties of explicit {\em
decodings} from translations to source terms.",
}
@techreport{BRICS-RS-99-42,
author = 	 "Nestmann, Uwe and Pierce, Benjamin C.",
title = 	 "Decoding Choice Encodings",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-42",
month = 	 dec,
note = 	 "ii+62~pp. Appears in {\em Journal of
Information and Computation}, 163:1--59, 2000.
An extended abstract appeared in " # lncs1119 #
", pages 179--194",
abstract = 	 "We study two encodings of the {\em
asynchronous $\pi$-calculus} with {\em
input-guarded choice} into its choice-free
fragment. One encoding is divergence-free, but
refines the atomic commitment of choice into
atomicity, but introduces divergence. The
divergent encoding is fully abstract with
respect to weak bisimulation, but the more
natural divergence-free encoding is not.
Instead, we show that it is fully abstract with
respect to {\it coupled simulation}, a slightly
coarser---but still coinductively
defined---equivalence that does not enforce
bisimilarity of internal branching decisions.
The correctness proofs for the two choice
encodings introduce a novel proof technique
exploiting the properties of explicit {\em
decodings} from translations to source terms.
",
}
@techreport{BRICS-RS-99-41,
author = 	 "Bodentien, Nicky O. and Vestergaard, Jacob and
Friis, Jakob and Kristoffersen, K{\aa}re J. and
Larsen, Kim G.",
title = 	 "Verification of State/Event Systems by
Quotienting",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-41",
month = 	 dec,
note = 	 "17~pp. Presented at {\em Nordic Workshop in
Programming Theory}, Uppsala, Sweden, October
6--8, 1999",
abstract = 	 "A rather new approach towards compositional
verification of concurrent systems is the
quotient technique, where components are
gradually removed from the concurrent system
while transforming the specification
accordingly. When the intermediate
specifications can be kept small, the state
explosion problem is avoided and there are
systems with an interleaving semantics,
including real-time systems. This paper extends
the quotienting approach to deal with a
synchronous framework in the shape of
state/event systems with acyclic dependencies.
A state/event system is a concurrent system
with a set of interdependent components
operating synchronously according to stimuli
provided by an environment. We introduce Left
Restricting state/event systems as a key notion
for state/event systems with acyclic
dependencies. Two families of modal logics,
$\mathcal{L}$ and $\mathcal{M}$, specifically
designed for expressing reachability properties
of dependency closed and not dependency closed
subsystems are introduced. Two quotient
constructions for moving components from
dependency closed and not dependency closed
subsystems into the specification are presented
and their correctness are justified in a formal
proof. Furthermore, heuristics for minimizing
formulae are proposed and the techniques are
demonstrated on a Duplo train example",
}

@techreport{BRICS-RS-99-40,
author = 	 "Grobauer, Bernd and Yang, Zhe",
title = 	 "The Second {F}utamura Projection for
Type-Directed Partial Evaluation",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-40",
month = 	 nov,
note = 	 "44~pp. Extended version of an article
appearing in " # acmpepm00 # ", pages 22--32. A
revised and extended version appears in {\em
Higher-Order and Symbolic Computation}
14(2--3):173--219 (2001) and " # alsoasrs #
"RS-00-44",
abstract = 	 "A generating extension of a program
specializes it with respect to some specified
part of the input. A generating extension of a
program can be formed trivially by applying a
partial evaluator to the program; the second
Futamura projection describes the automatic
generation of non-trivial generating extensions
by applying a partial evaluator to itself with
respect to the programs.

We derive an ML implementation of the second
Futamura projection for Type-Directed Partial
Evaluation (TDPE). Due to the differences
between traditional', syntax-directed partial
evaluation and TDPE, this derivation involves
several conceptual and technical steps. These
include a suitable formulation of the second
Futamura projection and techniques for making
TDPE amenable to self-application. In the
context of the second Futamura projection, we
also compare and relate TDPE with conventional
offline partial evaluation.

We demonstrate our technique with several
examples, including compiler generation for
Tiny, a prototypical imperative language.",
}
@techreport{BRICS-RS-99-39,
author = 	 "Rizzi, Romeo",
title = 	 "On the {S}teiner Tree $\frac {3}{2}$-Approximation for Quasi-Bipartite
Graphs",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-39",
month = 	 nov,
note = 	 "6~pp",
keywords = 	 "{S}teiner tree, local search, approximation
algorithm, bit scaling",
abstract = 	 "Let $G=(V,E)$ be an undirected simple graph
and $w:E\mapsto{\rm I\!R}_+$ be a non-negative
weighting of the edges of $G$. Assume $V$ is
partitioned as $R\cup X$. A {\em Steiner tree}
is any tree $T$ of $G$ such that every node in
$R$ is incident with at least one edge of $T$.
The {\em metric Steiner tree problem} asks for
a Steiner tree of minimum weight, given that
$w$ is a metric. When $X$ is a stable set of
$G$, then $(G,R,X)$ is called {\em
quasi-bipartite}. In a SODA~'99 paper,
Rajagopalan and Vazirani introduced the notion
of quasi-bipartiteness and gave a $(\frac {3}{2}+\epsilon)$ approximation algorithm for
the metric Steiner tree problem, when $(G,R,X)$
is quasi-bipartite. In this paper, we simplify
and strengthen the result of Rajagopalan and
Vazirani. We also show how classical bit
scaling techniques can be adapted to the design
of approximation algorithms",
}

@techreport{BRICS-RS-99-38,
author = 	 "Rizzi, Romeo",
title = 	 "Linear Time Recognition of $P_4$-Indifferent
Graphs",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-38",
month = 	 nov,
note = 	 "11~pp. Appears in {\em Discrete Mathematics},
239(1--3):161--169, 2001",
keywords = 	 "$P_4$-indifference, linear time, recognition
modular decomposition",
abstract = 	 "A simple graph is $P_4${\em-indifferent} if it
admits a total order $<$ on its nodes such that
every chordless path with nodes $a,b,c,d$ and
edges $ab,bc,cd$ has $ab>c>d$.
$P_4$-indifferent graphs generalize indifferent
graphs and are perfectly orderable. Recently,
Ho{\a}ng, Maffray and Noy gave a
characterization of $P_4$-indifferent graphs in
terms of forbidden induced subgraphs. We
clarify their proof and describe a linear time
algorithm to recognize $P_4$-indifferent
graphs. When the input is a $P_4$-indifferent
graph, then the algorithm computes an order $<$
as above",
}
@techreport{BRICS-RS-99-37,
author = 	 "Jord{\'a}n, Tibor",
title = 	 "Constrained Edge-Splitting Problems",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-37",
month = 	 nov,
note = 	 "23~pp. A preliminary version with the title
{\em Edge-Splitting Problems with Demands}
appeared in " # lncs1610 # ", pages 273--288",
abstract = 	 "Splitting off two edges $su, sv$ in a graph
$G$ means deleting $su,sv$ and adding a new
edge $uv$. Let $G=(V+s,E)$ be
$k$-edge-connected in $V$ ($k\geq 2$) and let
$d(s)$ be even. Lov{\'a}sz proved that the
edges incident to $s$ can be split off in pairs
in a such a way that the resulting graph on
vertex set $V$ is $k$-edge-connected. In this
paper we investigate the existence of such
complete splitting sequences when the set of
split edges has to meet additional
requirements. We prove structural properties of
the set of those pairs $u,v$ of neighbours of
$s$ for which splitting off $su,sv$ destroys
$k$-edge-connectivity. This leads to a new
method for solving problems of this
type.\bibpar

By applying this method we obtain a short proof
for a recent result of Nagamochi and Eades on
planarity-preserving complete splitting
sequences and prove the following new results:
let $G$ and $H$ be two graphs on the same set
$V+s$ of vertices and suppose that their sets
of edges incident to $s$ coincide. Let $G$
($H$) be $k$-edge-connected
($l$-edge-connected, respectively) in $V$ and
let $d(s)$ be even. Then there exists a pair
$su,sv$ which can be split off in both graphs
preserving $k$-edge-connectivity
($l$-edge-connectivity, resp.) in $V$, provided
$d(s)\geq 6$. If $k$ and $l$ are both even then
such a pair always exists. Using these
edge-splitting results and the polymatroid
intersection theorem we give a polynomial
algorithm for the problem of simultaneously
augmenting the edge-connectivity of two graphs
by adding a (common) set of new edges of
(almost) minimum size",
}

@techreport{BRICS-RS-99-36,
author = 	 "Cattani, Gian Luca and Winskel, Glynn",
title = 	 "Presheaf Models for {\bf CCS}-like Languages",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-36",
month = 	 nov,
note = 	 "ii+46~pp",
abstract = 	 "The aim of this paper is to harness the
mathematical machinery around presheaves for
the purposes of process calculi. Joyal, Nielsen
and Winskel proposed a general definition of
bisimulation from open maps. Here we show that
open-map bisimulations within a range of
presheaf models are congruences for a general
process language, in which {CCS} and related
languages are easily encoded. The results are
then transferred to traditional models for
processes. By first establishing the congruence
results for presheaf models, abstract, general
proofs of congruence properties can be provided
and the awkwardness caused through traditional
models not always possessing the cartesian
liftings, used in the break-down of process
operations, are side-stepped. The abstract
results are applied to show that hereditary
history-preserving bisimulation is a congruence
for {CCS}-like languages to which is added a
refinement operator on event structures as
proposed by van Glabbeek and Goltz",
}

@techreport{BRICS-RS-99-35,
author = 	 "Jord{\'a}n, Tibor and Szigeti, Zolt{\'a}n",
title = 	 "Detachments Preserving Local Edge-Connectivity
of Graphs",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-35",
month = 	 nov,
note = 	 "16~pp",
abstract = 	 "Let $G=(V+s,E)$ be a graph and let ${\cal S}=(d_1,...,d_p)$ be a set of positive integers
with $\sum d_j=d(s)$. An $\cal S$-detachment
splits $s$ into a set of $p$ independent
vertices $s_1,...,s_p$ with $d(s_j)=d_j$,
$1\leq j\leq p$. Given a requirement function
$r(u,v)$ on pairs of vertices of $V$, an $\cal S$-detachment is called $r$-admissible if the
detached graph $G'$ satisfies $\lambda_ {G'}(x,y)\geq r(x,y)$ for every pair $x,y\in V$. Here $\lambda_H(u,v)$ denotes the local
edge-connectivity between $u$ and $v$ in graph
$H$.\bibpar

We prove that an $r$-admissible $\cal S$-detachment exists if and only if (a)
$\lambda_G(x,y)\geq r(x,y)$, and (b) $\lambda_ {G-s}(x,y)\geq r(x,y)-\sum\lfloor d_j/2 \rfloor$ hold for every $x,y\in V$.\bibpar

The special case of this characterization when
$r(x,y)=\lambda_G(x,y)$ for each pair in $V$
was conjectured by B.~Fleiner. Our result is a
common generalization of a theorem of W.~Mader
on edge splittings preserving local
edge-connectivity and a result of B.~Fleiner on
detachments preserving global
edge-connectivity. Other corollaries include
previous results of L.~Lov{\'a}sz and
C.~J.~St.~A. Nash-Williams on edge splittings
and detachments, respectively. As a new
application, we extend a theorem of A.~Frank on
local edge-connectivity augmentation to the
case when stars of given degrees are added",
}

@techreport{BRICS-RS-99-34,
author = 	 "Rodler, Flemming Friche",
title = 	 "Wavelet Based {3D} Compression for Very Large
Volume Data Supporting Fast Random Access",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-34",
month = 	 oct,
note = 	 "36~pp. Extended version of paper appearing in
" # ieeepg99 # ", pages 108--117",
abstract = 	 "We propose a wavelet based method for
compressing volume\-tric data with little loss
in quality. The method supports fast random
compressed volume. Such a method is important
since storing and visualising very large
volumes impose heavy demands on internal memory
and external storage facilities making it
accessible only to users with huge and
expensive computers. This problem is not likely
to become less in the future. Experimental
results on the CT dataset of the Visible Human
have shown that our method provides very high
compression rates with fairly fast random
access",
}
@techreport{BRICS-RS-99-33,
author = 	 "Aceto, Luca and {\'E}sik, Zolt{\'a}n and
Ing{\'o}lfsd{\'o}ttir, Anna",
title = 	 "The Max-Plus Algebra of the Natural Numbers
has no Finite Equational Basis",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-33",
month = 	 oct,
note = 	 "25~pp. A revised version of this paper appears
{\em Theoretical Computer Science}
293(1):169--188, 17 January 2003.",
keywords = 	 "Equational logic, varieties, complete
axiomatisations, exponential time complexity",
abstract = 	 "This paper shows that the collection of
identities which hold in the algebra {\bf N} of
the natural numbers with constant zero, and
binary operations of sum and maximum is not
finitely based. Moreover, it is proven that,
for every $n$, the equations in at most $n$
variables that hold in {\bf N} do not form an
equational basis. As a stepping stone in the
proof of these facts, several results of
independent interest are obtained. In
particular, explicit descriptions of the free
algebras in the variety generated by {\bf N}
are offered. Such descriptions are based upon a
geometric characterisation of the equations
that hold in {\bf N}, which also yields that
the equational theory of {\bf N} is decidable
in exponential time",
}
@techreport{BRICS-RS-99-32,
author = 	 "Aceto, Luca and Laroussinie, Fran{\c c}ois",
title = 	 "Is your Model Checker on Time? --- On the
Complexity of Model Checking for Timed Modal
Logics",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-32",
month = 	 oct,
note = 	 "11~pp. Appears in " # lncs1672 # ", pages
125--136",
abstract = 	 "This paper studies the structural complexity
of model checking for (variations on) the
specification formalisms used in the tools CMC
and {\sc Uppaal}, and fragments of a timed
alternation-free $\mu$-calculus. For each of
the logics we study, we characterize the
computational complexity of model checking, as
well as its specification and program
complexity, using timed automata as our system
model",
}

@techreport{BRICS-RS-99-31,
author = 	 "Kohlenbach, Ulrich",
title = 	 "Foundational and Mathematical Uses of Higher
Types",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-31",
month = 	 sep,
note = 	 "34~pp. A revised version appears in W.~Sieg et
al. (eds.), {\em Reflections on the Foundations
of Mathematics}. Essays in Honor of Solomon
Feferman, Lecture Notes in Logic 15, A. K.
Peters, Ltd., pp.\ 92--116, 2002.",
abstract = 	 "In this paper we develop mathematically strong
systems of analysis in higher types which,
nevertheless, are proof-theoretically weak,
i.e.\
conservative over elementary resp.
primitive recursive arithmetic. These systems
are based on non-collapsing hierarchies
$(\Phi_n$-WKL$_+, \ \Psi_n$-WKL$_+)$ of
principles which generalize (and for $n=0$
coincide with) the so-called weak' K{\"o}nig's
lemma WKL (which has been studied extensively
in the context of second order arithmetic) to
logically more complex tree predicates. Whereas
the second order context used in the program of
reverse mathematics requires an encoding of
higher analytical concepts like continuous
functions $F:X\rightarrow Y$ between Polish
spaces $X,Y$, the more flexible language of our
systems allows to treat such objects directly.
This is of relevance as the encoding of $F$
used in reverse mathematics tacitly yields a
constructively enriched notion of continuous
functions which e.g. for $F:{\mathord{\rm I\mkern-3.1mu N}}^{{\mathord{\rm I\mkern-3.1mu N}}}\rightarrow{\mathord{\rm I\mkern-3.1mu N}}$
can be seen (in our higher order context) to be
equivalent to the existence of a continuous
modulus of pointwise continuity. For the direct
representation of $F$ the existence of such a
modulus is independent even of full arithmetic
in all finite types E-PA$^{\omega}$ plus
quantifier-free choice, as we show using a
priority construction due to L. Harrington. The
usual WKL-based proofs of properties of $F$
given in reverse mathematics make use of the
enrichment provided by codes of $F$, and WKL
does not seem to be sufficient to obtain
similar results for the direct representation
of $F$ in our setting. However, it turns out
that $\Psi_1$-WKL$_+$ {\bf is} sufficient. \\
Our conservation results for $(\Phi_n$-WKL$_+, \ \Psi_n$-WKL$_+)$ are proved via a new
elimination result for a strong non-standard
principle of uniform $\Sigma^0_1$-boundedness
which we introduced in 1996 and which implies
the WKL-extensions studied in this paper",
}
@techreport{BRICS-RS-99-30,
author = 	 "Aceto, Luca and Fokkink, Willem Jan and
Verhoef, Chris",
title = 	 "Structural Operational Semantics",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-30",
month = 	 sep,
note = 	 "128~pp. Appears in " # ehpa # ", pages
197--292",
abstract = 	 "This paper offers an overview of the current
state of the development of the theory of
Structural Operational Semantics (SOS), with
emphasis on its applications to process
algebra. It focuses on five aspects of SOS,
viz.~the meaning of Transition System
Specifications (TSSs) with predicates and
negative premises, conservative extension
results for TSSs, congruence formats,
many-sorted higher-order extensions and
connections with denotational semantics",
}
@techreport{BRICS-RS-99-29,
author = 	 "Riis, S{\o}ren",
title = 	 "A Complexity Gap for Tree-Resolution",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-29",
month = 	 sep,
note = 	 "33~pp",
keywords = 	 "Logical aspects of Complexity, Propositional
proof complexity, Resolution proofs",
abstract = 	 "It is shown that any sequence $\psi_n$ of
tautologies which expresses the validity of a
fixed combinatorial principle either is
easy'' i.e. has polynomial size
tree-resolution proofs or is difficult'' i.e
requires exponential size tree-resolution
proofs. It is shown that the class of
tautologies which are hard (for
tree-resolution) is identical to the class of
tautologies which are based on combinatorial
principles which are violated for infinite
sets. Actually it is shown that the
gap-phenomena is valid for tautologies based on
infinite mathematical theories (i.e.\ not just
based on a single proposition). \bibpar

We clarify the link between translating
combinatorial principles (or more general
statements from predicate logic) and the recent
idea of using the symmetrical group to generate
problems of propositional logic. \bibpar

Finally, we show that is undecidable whether a
sequence $\psi_n$ (of the kind we consider) has
polynomial size tree-resolution proofs or
requires exponential size tree-resolution
proofs. Also we show that the degree of the
polynomial in the polynomial size (in case it
exists) is non-recursive, but semi-decidable.",
}

@techreport{BRICS-RS-99-28,
author = 	 "Hildebrandt, Thomas Troels",
title = 	 "A Fully Abstract Presheaf Semantics of {SCCS}
with Finite Delay",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-28",
month = 	 sep,
note = 	 "37~pp. Appears in " # entcsctcs99 # ", 25~pp",
abstract = 	 "We present a presheaf model for the
observation of {\em infinite} as well as finite
computations. We apply it to give a {\em
denotational} semantics of SCCS with finite
delay, in which the meanings of recursion are
given by {\em final} coalgebras and meanings of
finite delay by {\em initial} algebras of the
process equations for delay. This can be viewed
as a first step in representing {\em fairness}
in presheaf semantics. We give a concrete
representation of the presheaf model as a
category of {\em generalised synchronisation
trees} and show that it is coreflective in a
category of {\em generalised transition
systems}, which are a special case of the
general transition systems of Hennessy and
Stirling. The open map bisimulation is shown to
coincide with the {\em extended bisimulation}
of Hennessy and Stirling. Finally we formulate
Milners operational semantics of SCCS with
finite delay in terms of generalised transition
systems and prove that the presheaf semantics
is {\em fully abstract} with respect to
extended bisimulation",
}
@techreport{BRICS-RS-99-27,
author = 	 "Danvy, Olivier and Schultz, Ulrik P.",
title = 	 "Lambda-Dropping: Transforming Recursive
Equations into Programs with Block Structure",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-27",
month = 	 sep,
note = 	 "57~pp. Appears in {\em Theoretical Computer
Science}, 248(1--2):243--287, November 2000.
This revised report supersedes the earlier
BRICS report RS-98-54",
abstract = 	 "Lambda-lifting a block-structured program
transforms it into a set of recursive
equations. We present the symmetric
transformation: lambda-dropping.
Lambda-dropping a set of recursive equations
restores block structure and lexical
scope.\bibpar

For lack of block structure and lexical scope,
recursive equations must carry around all the
parameters that any of their callees might
possibly need. Both lambda-lifting and
lambda-dropping thus require one to compute
Def/Use paths:
\begin{itemize}
\item for lambda-lifting: each of the functions
occurring in the path of a free variable is
passed this variable as a parameter;

\item for lambda-dropping: parameters which are
used in the same scope as their definition do
not need to be passed along in their path.
\end{itemize}

A program whose blocks have no free variables
is scope-insensitive. Its blocks are then free
to float (for lambda-lifting) or to sink (for
lambda-dropping) along the vertices of the
scope tree. \bibpar

Our primary application is partial evaluation.
Indeed, many partial evaluators for procedural
programs operate on recursive equations. To
this end, they lambda-lift source programs in a
pre-processing phase. But often, partial
evaluators [automatically] produce residual
recursive equations with dozens of parameters,
which most compilers do not handle efficiently.
We solve this critical problem by
lambda-dropping residual programs in a
post-processing phase, which significantly
improves both their compile time and their run
time. \bibpar

Lambda-lifting has been presented as an
intermediate transformation in compilers for
functional languages. We study lambda-lifting
and lambda-dropping per se, though
lambda-dropping also has a use as an
intermediate transformation in a compiler: we
noticed that lambda-drop\-ping a program
corresponds to transforming it into the
functional representation of its optimal SSA
form. This observation actually led us to
substantially improve our PEPM~'97 presentation
of lambda-dropping",
}
@techreport{BRICS-RS-99-26,
author = 	 "Henriksen, Jesper G.",
title = 	 "An Expressive Extension of {TLC}",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-26",
month = 	 sep,
note = 	 "20~pp. Appears in {\em International Journal
of Foundations of Computer Science},
13(3):341--360, 2002. Conference version in " #
lncs1742 # ", pages 126--138",
abstract = 	 "A temporal logic of causality (TLC) was
introduced by Alur, Penczek and Peled. It is
basically a linear time temporal logic
interpreted over Mazurkiewicz traces which
allows quantification over causal chains.
Through this device one can directly formulate
causality properties of distributed systems. In
this paper we consider an extension of TLC by
strengthening the chain quantification
operators. We show that our logic TLC$^*$ adds
to the expressive power of TLC. We do so by
defining an Ehrenfeucht-Fra{\"\i}ss{\'e} game
to capture the expressive power of TLC. We then
exhibit a property and by means of this game
prove that the chosen property is not definable
in TLC. We then show that the same property is
definable in TLC$^*$. We prove in fact the
stronger result that TLC$^*$ is expressively
stronger than TLC exactly when the dependency
relation associated with the underlying trace
alphabet is not transitive.",
}
@techreport{BRICS-RS-99-25,
author = 	 "Brodal, Gerth St{\o}lting and Pedersen,
Christian N. S.",
title = 	 "Finding Maximal Quasiperiodicities in
Strings",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-25",
month = 	 sep,
note = 	 "20~pp. Appears in " # lncs1848 # ", pages
397--411",
abstract = 	 "Apostolico and Ehrenfeucht defined the notion
of a maximal quasiperiodic substring and gave
an algorithm that finds all maximal
quasiperiodic substrings in a string of
length~$n$ in time $O(n \log^2 n)$. In this
paper we give an algorithm that finds all
maximal quasiperiodic substrings in a string of
length~$n$ in time $O(n\log n)$ and
space~$O(n)$. Our algorithm uses the suffix
tree as the fundamental data structure combined
with efficient methods for merging and
performing multiple searches in search trees.
Besides finding all maximal quasiperiodic
substrings, our algorithm also marks the nodes
in the suffix tree that have a superprimitive
path-label",
}
@techreport{BRICS-RS-99-24,
author = 	 "Aceto, Luca and Fokkink, Willem Jan and
Verhoef, Chris",
title = 	 "Conservative Extension in Structural
Operational Semantics",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-24",
month = 	 sep,
note = 	 "23~pp. Appears in the {\em Bulletin of the
European Association for Theoretical Computer
Science}, 70:110--132, 1999",
abstract = 	 "Over and over again, process calculi such as
CCS, CSP, and ACP have been extended with new
features, and the Transition System
Specifications (TSSs) that provide the
operational semantics for these process
algebras were extended with transition rules to
describe these features. A question that arises
naturally is whether or not the original and
the extended TSS induce the same transitions in
the original domain. Usually it is desirable
that an extension is operationally
conservative, meaning that the provable
transitions for an original term are the same
both in the original and in the extended TSS.
This paper contains an exposition of existing
conservative extension formats for Structural
Operational Semantics, and their applications
with respect to term rewriting systems and
completeness of axiomatizations",
}
@techreport{BRICS-RS-99-23,
author = 	 "Danvy, Olivier and Dzafic, Belmina and
Pfenning, Frank",
title = 	 "On proving syntactic properties of {CPS}
programs",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-23",
month = 	 aug,
note = 	 "14~pp. Appears in " # entcshoots99 # ", pages
19--31",
abstract = 	 "Higher-order program transformations raise new
challenges for proving properties of their
first-order proof techniques. In this work, we
consider (1) the one-pass''
continuation-passing style (CPS)
transformation, which is second-order, and (2)
the occurrences of parameters of continuations
in its output. To this end, we specify the
one-pass CPS transformation relationally and we
use the proof technique of logical relations",
}
@techreport{BRICS-RS-99-22,
author = 	 "Aceto, Luca and {\'E}sik, Zolt{\'a}n and
Ing{\'o}lfsd{\'o}ttir, Anna",
title = 	 "On the Two-Variable Fragment of the Equational
Theory of the Max-Sum Algebra of the Natural
Numbers",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-22",
month = 	 aug,
note = 	 "22~pp. Appears in " # lncs1770 # ", pages
267--278",
abstract = 	 "This paper shows that the collection of
identities in two variables which hold in the
algebra {\bf N} of the natural numbers with
constant zero, and binary operations of sum and
maximum does not have a finite equational
axiomatization. This gives an alternative proof
of the non-existence of a finite basis for {\bf
N}---a result previously obtained by the
authors. As an application of the main theorem,
it is shown that the language of Basic Process
Algebra (over a singleton set of actions), with
or without the empty process, has no finite
equational axiomatization modulo trace
equivalence",
}
@techreport{BRICS-RS-99-21,
author = 	 "Danvy, Olivier",
title = 	 "An Extensional Characterization of
Lambda-Lifting and Lambda-Dropping",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-21",
month = 	 aug,
note = 	 "13~pp. Extended version of an article
appearing in " # lncs1722 # ", pages 241--250.
This report supersedes the earlier BRICS report
RS-98-2",
abstract = 	 "Lambda-lifting and lambda-dropping
respectively transform a block-structured
functional program into recursive equations and
vice versa. Lambda-lifting was developed in the
early 80's, whereas lambda-dropping is more
recent. Both are split into an analysis and a
transformation. Published work, however, has
only concentrated on the analysis parts. We
focus here on the transformation parts and more
precisely on their correctness, which appears
never to have been proven. To this end, we
define extensional versions of lambda-lifting
and lambda-dropping and establish their
correctness with respect to a least fixed-point
semantics",
}
@techreport{BRICS-RS-99-20,
author = 	 "Kohlenbach, Ulrich",
title = 	 "A Note on {S}pector's Quantifier-Free Rule of
Extensionality",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-20",
month = 	 aug,
note = 	 "5~pp. Appears in {\em Archive for Mathematical
Logic}, 40:89--92, 2001",
abstract = 	 "In this note we show that the so-called weakly
extensional arithmetic in all finite types,
which is based on a quantifier-free rule of
extensionality due to C. Spector and which is
of significance in the context of G{\"o}del's
functional interpretation, does not satisfy the
deduction theorem for additional axioms. This
holds already for $\Pi^0_1$-axioms. Previously,
only the failure of the stronger deduction
theorem for deductions from (possibly open)
assumptions (with parameters kept fixed) was
known.",
}
@techreport{BRICS-RS-99-19,
author = 	 "Jurdzi{\'n}ski, Marcin and Nielsen, Mogens",
title = 	 "Hereditary History Preserving Bisimilarity is
Undecidable",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-19",
month = 	 jun,
note = 	 "18~pp. An extended abstract appears in " #
lncs1770 # ", pages 358--369",
abstract = 	 "We show undecidability of hereditary history
preserving bisimilarity for finite asynchronous
transition systems by a reduction from the
halting problem of deterministic 2-counter
machines. To make the proof more transparent we
introduce an intermediate problem of checking
domino bisimilarity for origin constrained
tiling systems. First we reduce the halting
problem of deterministic 2-counter machines to
origin constrained domino bisimilarity. Then we
show how to model domino bisimulations as
hereditary history preserving bisimulations for
finite asynchronous transitions systems. We
also argue that the undecidability result holds
for finite 1-safe Petri nets, which can be seen
as a proper subclass of finite asynchronous
transition systems",
}
@techreport{BRICS-RS-99-18,
author = 	 "M{\"o}ller, M. Oliver and Rue{\ss}, Harald",
title = 	 "Solving Bit-Vector Equations of Fixed and
Non-Fixed Size",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-18",
month = 	 jun,
note = 	 "18~pp. Revised version of an article appearing
under the title {\em Solving Bit-Vector
Equations\/} in " # lncs1522 # ", pages
36--48",
abstract = 	 "This report is concerned with solving
equations on fixed and non-fixed size
bit-vector terms. We define an equational
transformation system for solving equations on
terms where all sizes of bit-vectors and
extraction positions are known. This
transformation system suggests a generalization
for dealing with bit-vectors of unknown size
and unknown extraction positions. Both solvers
adhere to the principle of splitting
bit-vectors only on demand, thereby making them
quite effective in practice",
}

@techreport{BRICS-RS-99-17,
author = 	 "Filinski, Andrzej",
title = 	 "A Semantic Account of Type-Directed Partial
Evaluation",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-17",
month = 	 jun,
note = 	 "23~pp. Appears in " # lncs1702 # ", pages
378--395",
abstract = 	 "We formally characterize partial evaluation of
functional programs as a normalization problem
in an equational theory, and derive a
type-based normalization-by-evaluation
algorithm for computing normal forms in this
setting. We then establish the correctness of
this algorithm using a semantic argument based
on Kripke logical relations. For simplicity,
the results are stated for a non-strict, purely
functional language; but the methods are
directly applicable to stating and proving
correctness of type-directed partial evaluation
in ML-like languages as well",
}
@techreport{BRICS-RS-99-16,
author = 	 "Lyngs{\o}, Rune B. and Pedersen, Christian N.
S.",
title = 	 "Protein Folding in the {2D HP} Model",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-16",
month = 	 jun,
note = 	 "15~pp. Appears in {\em Proceedings of the 1st
Journ{\'e}es Ouvertes: Biologie, Informatique
et Math{\'e}matiques (JOBIM 2000)}",
abstract = 	 "We study folding algorithms in the two
dimensional Hydrophobic-Hydrophilic model (2D
HP model) for protein structure formation. We
consider three generalizations of the best
known approximation algorithm. We show that two
of the generalizations do not improve the worst
case approximation ratio. The third
generalization seems to be better, and the
analysis of its approximation ratio leads to an
interesting combinatorial problem",
}
@techreport{BRICS-RS-99-15,
author = 	 "Lyngs{\o}, Rune B. and Zuker, Michael and
Pedersen, Christian N. S.",
title = 	 "An Improved Algorithm for {RNA} Secondary
Structure Prediction",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-15",
month = 	 may,
note = 	 "24~pp. An alloy of two articles appearing in "
# acmrecomb99 # ", pages 260--267, and {\em
Bioinformatics}, 15(6):440--445, June 1999",
abstract = 	 "Though not as abundant in known biological
processes as proteins, RNA molecules serve as
more than mere intermediaries between DNA and
proteins, e.g.~as catalytic molecules.
Furthermore, RNA secondary structure prediction
based on free energy rules for stacking and
loop formation remains one of the few major
breakthroughs in the field of structure
prediction. We present a new method to evaluate
all possible internal loops of size at most $k$
in an RNA sequence, $s$, in time $O(k|s|^2)$;
this is an improvement from the previously used
method that uses time $O(k^2|s|^2)$. For
unlimited loop size this improves the overall
complexity of evaluating RNA secondary
structures from $O(|s|^4)$ to $O(|s|^3)$ and
the method applies equally well to finding the
optimal structure and calculating the
equilibrium partition function. We use our
method to examine the soundness of setting $k = 30$, a commonly used heuristic",
}
@techreport{BRICS-RS-99-14,
author = 	 "Fiore, Marcelo P. and Cattani, Gian Luca and
Winskel, Glynn",
title = 	 "Weak Bisimulation and Open Maps",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-14",
month = 	 may,
note = 	 "22~pp. Appears in " # lics14 # ", pages
67--76",
abstract = 	 "A general treatment of weak bisimulation and
observation congruence on presheaf models is
presented. It extends previous work on
bisimulation from open maps and answers a
fundamental question there. The consequences of
the general theory are derived for two key
models for concurrency, the interleaving model
of synchronisation trees and the independence
model of labelled event structures. Starting
with a hiding'' functor from a category of
paths to observable paths, it is shown how to
derive a monad on the category of presheaves
over the category of paths. The Kleisli
category of the monad is shown to be equivalent
to that of presheaves with weak morphisms,
which roughly are only required to preserve
observable paths. The monad is defined through
an intermediary view of processes as bundles in
{\bf Cat}. This view, which seems important in
its own right, leads directly to a hiding
operation on processes; when the hiding
operation is translated back to presheaves
monad is shown to automatically preserve
open-map bisimulation, generalising the
expectation that strongly bisimilar processes
should be weakly bisimilar. However, two
alternative general ways to define weak
bisimulation and observation congruence (in the
Kleisli category itself or in the presheaf
category after application of the monad) do not
coincide in general. However a necessary and
sufficient condition for their equivalence is
proved; from this it is sufficient to show a
fill-in condition applies to category of paths
with weak morphisms. The fill-in condition is
shown to hold for the two traditional models,
synchronisation trees and event structures",
}
@techreport{BRICS-RS-99-13,
author = 	 "Pagh, Rasmus",
title = 	 "Hash and Displace: Efficient Evaluation of
Minimal Perfect Hash Functions",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-13",
month = 	 may,
note = 	 "11~pp. A short version appears in " # lncs1663
# ", pages 49--54",
abstract = 	 "A new way of constructing (minimal) perfect
hash functions is described. The technique
with resolving buckets in two-level hashing
schemes. Evaluating a hash function requires
just one multiplication and a few additions
apart from primitive bit operations. The number
of accesses to memory is two, one of which is
to a fixed location. This improves the probe
performance of previous minimal perfect hashing
schemes, and is shown to be optimal. The hash
function description (program'') for a set of
size $n$ occupies $O(n)$ words, and can be
constructed in expected $O(n)$ time",
}
@techreport{BRICS-RS-99-12,
author = 	 "Brodal, Gerth St{\o}lting and Lyngs{\o}, Rune
B. and Pedersen, Christian N. S. and Stoye,
Jens",
title = 	 "Finding Maximal Pairs with Bounded Gap",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-12",
month = 	 apr,
note = 	 "31~pp. Appears in " # lncs1645 # ", pages
134--149. Journal version in {\em Journal of
Discrete Algorithms}, 1:77--104, 2000",
abstract = 	 "A pair in a string is the occurrence of the
same substring twice. A pair is maximal if the
two occurrences of the substring cannot be
extended to the left and right without making
them different. The gap of a pair is the number
of characters between the two occurrences of
the substring. In this paper we present methods
for finding all maximal pairs under various
constraints on the gap. In a string of
length~$n$ we can find all maximal pairs with
gap in an upper and lower bounded interval in
time $O(n \log n + z)$ where~$z$ is the number
of reported pairs. If the upper bound is
removed the time reduces to~$O(n + z)$. Since a
tandem repeat is a pair where the gap is zero,
our methods can be seen as a generalization of
finding tandem repeats. The running time of our
methods equals the running time of well known
methods for finding tandem repeats",
}
@techreport{BRICS-RS-99-11,
author = 	 "Kohlenbach, Ulrich",
title = 	 "On the Uniform Weak {K}{\"o}nig's Lemma",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-11",
month = 	 mar,
note = 	 "13~pp. An extended version appears in {\em
Annals of Pure and Applied Logic} (special
issue in honor of A.~S. Troelstra, 2001) vol.
114, pp. 103-116 (2002).",
abstract = 	 "The so-called weak K{\"o}nig's lemma WKL
asserts the existence of an infinite path $b$
in any infinite binary tree (given by a
representing function $f$). Based on this
principle one can formulate subsystems of
higher-order arithmetic which allow to carry
out very substantial parts of classical
mathematics but are $\Pi^0_2$-conservative over
primitive recursive arithmetic PRA (and even
weaker fragments of arithmetic). In 1992 we
established such conservation results relative
to finite type extensions PRA$^{\omega}$ of PRA
(together with a quantifier-free axiom of
choice schema). In this setting one can
consider also a uniform version UWKL of WKL
which asserts the existence of a functional
$\Phi$ which selects uniformly in a given
infinite binary tree $f$ an infinite path $\Phi f$ of that tree. This uniform version of WKL is
of interest in the context of explicit
mathematics as developed by S. Feferman. The
elimination process from 1992 actually can be
used to eliminate even this uniform weak
K{\"o}nig's lemma provided that PRA$^{\omega}$
only has a quantifier-free rule of
extensionality QF-ER instead of the full axioms
$(E)$ of extensionality for all finite types.
In this paper we show that in the presence of
$(E)$, UWKL is much stronger than WKL: whereas
WKL remains to be $\Pi^0_2$-conservative over
PRA, PRA$^{\omega}+(E)+$UWKL contains (and is
conservative over) full Peano arithmetic PA",
}
@techreport{BRICS-RS-99-10,
author = 	 "Riecke, Jon G. and Sandholm, Anders B.",
title = 	 "A Relational Account of Call-by-Value
Sequentiality",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-10",
month = 	 mar,
note = 	 "51~pp. To appear in {\em Information and
Computation}, LICS~'97 Special Issue. Extended
version of an article appearing in " # lics12 #
", pages 258--267. This report supersedes the
earlier BRICS report RS-97-41",
abstract = 	 "We construct a model for FPC, a purely
functional, sequential, call-by-value language.
The model is built from partial continuous
functions, in the style of Plotkin, further
constrained to be uniform with respect to a
class of logical relations. We prove that the
model is fully abstract",
}

@techreport{BRICS-RS-99-9,
author = 	 "Brabrand, Claus and M{\o}ller, Anders and
Sandholm, Anders B. and Schwartzbach, Michael
I.",
title = 	 "A Runtime System for Interactive Web
Services",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-9",
month = 	 mar,
note = 	 "21~pp. Appears in " # www8 # ", pages 313--323
and {\em Computer Networks}, 31:1391--1401,
1999",
abstract = 	 "Interactive web services are increasingly
Producing web services seems to require a
tremendous amount of laborious low-level coding
due to the primitive nature of CGI programming.
We present ideas for an improved runtime system
for interactive web services built on top of
CGI running on virtually every combination of
browser and HTTP/CGI server. The runtime system
has been implemented and used extensively in
$<$bigwig$>$, a tool for producing interactive
web services",
}

@techreport{BRICS-RS-99-8,
author = 	 "Havelund, Klaus and Larsen, Kim G. and Skou,
Arne",
title = 	 "Formal Verification of a Power Controller
Using the Real-Time Model Checker {\sc
{U}ppaal}",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-8",
month = 	 mar,
note = 	 "23~pp. Appears in " # lncs1601 # ", pages
277--298",
abstract = 	 "A real-time system for power-down control in
audio/video components is modeled and verified
using the real-time model checker {\sc Uppaal}.
The system is supposed to reside in an
audio/video component and control (read from
and write to) links to neighbor audio/video
components such as TV, VCR and remote--control.
In particular, the system is responsible for
the powering up and down of the component in
between the arrival of data, and in order to do
so in a safe way without loss of data, it is
essential that no link interrupts are lost.
Hence, a component system is a multitasking
system with hard real-time requirements, and we
present techniques for modeling time
consumption in such a multitasked, prioritized
system. The work has been carried out in a
collaboration between Aalborg University and
the audio/video company B\&O. By modeling the
system, 3 design errors were identified and
corrected, and the following verification
confirmed the validity of the design but also
revealed the necessity for an upper limit of
the interrupt frequency. The resulting design
has been implemented and it is going to be
incorporated as part of a new product line",
}
@techreport{BRICS-RS-99-7,
author = 	 "Winskel, Glynn",
title = 	 "Event Structures as Presheaves---{T}wo
Representation Theorems",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-7",
month = 	 mar,
note = 	 "16~pp. Appears in " # lncs1664 # ", pages
541--556",
abstract = 	 "The category of event structures is known to
embed fully and faithfully in the category of
presheaves over pomsets. Here a
characterisation of the presheaves represented
by event structures is presented. The proof
goes via a characterisation of the presheaves
represented by event structures when the
morphisms on event structures are strict'' in
that they preserve the partial order of causal
dependency",
}
@techreport{BRICS-RS-99-6,
author = 	 "Lyngs{\o}, Rune B. and Pedersen, Christian N.
S. and Nielsen, Henrik",
title = 	 "Measures on Hidden {M}arkov Models",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-6",
month = 	 feb,
note = 	 "27~pp. Appears in " # isbm99 # " as {\em
Metrics and similarity measures for hidden
Markov models}",
abstract = 	 "Hidden Markov models were introduced in the
beginning of the 1970's as a tool in speech
recognition. During the last decade they have
been found useful in addressing problems in
computational biology such as characterising
sequence families, gene finding, structure
prediction and phylogenetic analysis. In this
paper we propose several measures between
hidden Markov models. We give an efficient
algorithm that computes the measures for
profile hidden Markov models and discuss how to
extend the algorithm to other types of models.
We present an experiment using the measures to
compare hidden Markov models for three classes
of signal peptides",
}
@techreport{BRICS-RS-99-5,
author = 	 "Bradfield, Julian C. and Stevens, Perdita",
title = 	 "Observational Mu-Calculus",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-5",
month = 	 feb,
note = 	 "18~pp",
abstract = 	 "We propose an extended modal mu-calculus to
provide an assembly language' for modal logics
for real time, value-passing calculi, and other
extended models of computation",
}

@techreport{BRICS-RS-99-4,
author = 	 "Fr{\"o}schle, Sibylle B. and Hildebrandt,
Thomas Troels",
title = 	 "On Plain and Hereditary History-Preserving
Bisimulation",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-4",
month = 	 feb,
note = 	 "21~pp. Appears in " # lncs1672 # ", pages
354--365",
abstract = 	 "We investigate the difference between two
well-known notions of independence
bisimilarity, {\em history-preserving
bisimulation} and {\em hereditary
history-preserving bisimulation}. We
characterise the difference between the two
bisimulations in {\em trace-theoretical} terms,
advocating the view that the first is (just) a
bisimulation for {\em causality}, while the
second is a bisimulation for {\em concurrency}.
We explore the frontier zone between the two
notions by defining a {\em hierarchy} of
bounded backtracking bisimulations. Our goal is
to provide a stepping stone for the solution to
the intriguing open problem of whether
hereditary history-preserving bisimulation is
decidable or not. We prove that each of the
bounded bisimulations is decidable. However, we
also prove that the hierarchy is strict. This
rules out the possibility that decidability of
the general problem follows directly from the
special case. Finally, we give a non trivial
reduction solving the general problem for a
restricted class of systems and give pointers
}
@techreport{BRICS-RS-99-3,
author = 	 "Miltersen, Peter Bro",
title = 	 "Two Notes on the Computational Complexity of
One-Dimensional Sandpiles",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-3",
month = 	 feb,
note = 	 "8~pp",
abstract = 	 "We prove that the one-dimensional sandpile
prediction problem is in $\mbox{\rm\bf AC}^1$.
The best previously known upper bound on the
$\mbox{\rm\bf AC}^i$-scale was $\mbox{\bf AC}^2$. We also prove that the problem is not
in $\mbox{\rm\bf AC}^{1-\epsilon}$ for any
constant $\epsilon> 0$",
}

@techreport{BRICS-RS-99-2,
author = 	 "Damg{\aa}rd, Ivan B.",
title = 	 "An Error in the Mixed Adversary Protocol by
{F}itzi, {H}irt and {M}aurer",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-2",
month = 	 feb,
note = 	 "4~pp",
abstract = 	 "We point out an error in the protocol for
mixed adversaries and zero error from the
Crypto 98 paper by Fitzi, Hirt and Maurer. We
show that the protocol only works under a
stronger requirement on the adversary than the
one claimed. Hence the bound on the adversary's
corruption capability given there is not tight.
Subsequent work has shown, however, a new bound
which is indeed tight",
}

@techreport{BRICS-RS-99-1,
author = 	 "Jurdzi{\'n}ski, Marcin and Nielsen, Mogens",
title = 	 "Hereditary History Preserving Simulation is
Undecidable",
institution =  brics,
year = 	 1999,
type = 	 rs,
number = 	 "RS-99-1",
month = 	 jan,
note = 	 "15~pp",
abstract = 	 "We show undecidability of {\em hereditary
history preserving simulation} for finite
asynchronous transition systems by a reduction
from the {\em halting problem} of deterministic
Turing machines. To make the proof more
transparent we introduce an intermediate
problem of {\em deciding the winner} in {\em
domino snake games}. First we reduce the
halting problem of deterministic Turing
machines to domino snake games. Then we show
how to model a domino snake game by a {\em
hereditary history simulation game} on a pair
of finite {\em asynchronous transition
systems}",
}

`