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Theoretical Computer Science Forum
Spring 2005
This is a forum for talks on theoretical computer science organized by the
TCS Laboratory at HUT. We meet usually on Fridays at 2 p.m. in room
B353 in the Computer Science building.
For further information, please contact
Ilkka Niemelä.
Program
- 14.1.2005 Ammar Alkassar (University of Saarland, Germany):
Secure Object Identification - Or: How To Solve The
Chess-Grandmaster-Problem
Abstract:
Many applications of cryptographic identification protocols are vulnerable
against physical adversaries who perform real time attacks. For instance,
when identifying a physical object like a person requesting access to a
restricted area, common identification schemes can be bypassed by faithfully
relaying all messages between the communicating participants. This attack is
known as mafia fraud.
In my talk I will give an overview over different approaches to cope with
that fraud. One approach, the Probabilistic Channel Hopping system, solves
this problem by hiding the conversation channel between the participants.
The security of this approach is based on the assumption that an adversary
cannot efficiently relay all possible communication channels of the PCH
system in parallel.
Finally, I will present a concrete, zero-knowledge based identification
scheme and give a sketch of the proof idea.
- 3.3.2005 Prof. Michael Kaminski (Technion, Computer Science Department, Israel):
Invariance under Stuttering in a Temporal Logic of Actions
Note: the talk is at 14:15 in room A328
Abstract:
We show that some simple and useful stutter-invariant properties
definable in the language of Lamport's Simple Temporal Logic of
Actions (STLA) are not definable in STLA and present a natural
extension of STLA that allows one to express all stutter-invariant
properties definable in the language of STLA. The example of a
stutter-invariant property not expressible in STLA is based on a
version of Ehrenfeught-Fraisse games on temporal interpretations.
- 15.3.2005 Prof. Gerhard Brewka (Computer Science Institute,
University of Leipzig):
Component Systems: Bridging the Gap Between CP-Nets and Answer
Set Optimization
Note: the talk is at 9:15 in room A346
Abstract:
We introduce a flexible framework to specify problem solutions
(outcomes) and preferences among them. The proposal
combines ideas from answer-set programming (ASP), answer-set
optimization (ASO) and CP-nets. The problem domain is structured into
components. ASP techniques are used to specify values of
components, as well as global (inter-component) constraints among
these values. ASO methods are used to describe preferences among the
values of a component, CP-net techniques to represent inter-component
dependencies and corresponding preferences.
(Joint work with I. Niemelä and M. Truszczynski)
- 16.3.2005 Prof. Torsten Schaub (Universität Potsdam, Institut für
Informatik)
Platypus: A Platform for Distributed Answer Set Solving
Note: the talk is at 9:15 in room B353
Abstract:
We propose a model to manage the distributed computation of
answer sets within a general framework, incorporating a variety of
software and hardware architectures to allow its easy use with a diverse
cadre of computational elements. Starting from a generic algorithmic
scheme, we develop a platform for distributed answer set computation,
describe its current state of implementation, and give some experimental
results.
(Joint work with Jean Gressmann, Tomi Janhunen, Robert E. Mercer,
Richard Tichy, Sven Thiele.)
- 17.3.2005 Dipl.-Inf. Martin Gebser (Universität Potsdam, Institut für
Informatik)
Foundations of clause learning in ASP solving
Note: the talk is at 9:15 in room B353
Abstract:
Implication/conflict graphs form the fundament for enhancing
the Davis-Putnam procedure, dealing with propositional CNF formulas,
by clause learning. Basic algorithms of ASP solvers smodels and nomore
are very similar to the Davis-Putnam procedure. Nonetheless, there has
almost no work been done on customizing clause learning for ASP
solvers. This talk is dedicated to developing implication/conflict
graphs of smodels and nomore in a systematic manner by characterizing
inference rules in terms of clauses.
- 21.4.2005 Misa Keinänen:
Solving Boolean Equation Systems
Note: the talk is at 15:15 in room A106
Abstract:
Boolean equation systems are ordered sequences of Boolean
equations decorated with fixpoint operators. Boolean equation systems
provide a useful framework for computer aided verification because
various specification and verification problems can be encoded as the
problem of solving such fixpoint equation systems. In this thesis,
techniques for finding solutions to Boolean equation systems are
studied, and new methods for solving such systems are devised. An
approach to solve a general form Boolean equation system, which
simplifies the process of finding the solution by dividing the system
into more simple subsystems and solving these by optimized procedures,
is introduced and analyzed. New solution algorithms for disjunctive and
conjunctive classes of Boolean equation systems are presented, together
with an implementation and experimental evaluation of a solver for these
classes. A novel translation of the problem of solving a general form
Boolean equation system into the problem of finding a stable model of a
logic program is given. The translation allows to use an implementation
of an answer set programming framework, the Smodels system, to solve
Boolean equation systems. Experimental tests have been performed using
the presented approach, and these experiments indicate the effectiveness
of using answer set programming in this problem domain.
- 22.4.2005 Dr. Supriya Krishnamurthy
(Swedish Institute of Computer Science, Stockholm):
The Statistical Mechanics of Dynamic Peer-to-Peer Networks
Abstract:
In Peer-to-Peer (P2P) networks, every member of the
network has equal capability and responsibility in contrast to
traditional client/server network architectures. These networks
have been the subject of much current research, for their intrinsic
interest as well as their potential for being deployed in applications.
The membership of these networks is however dynamic, so that
both the network as well as the function it serves has to be robust
against constant change. Earlier studies of these systems have
either depended on simulations as the primary investigation tool, or on
establishing theoretical bounds. In this
talk, we present a different approach to looking at these networks
using a master-equation-based approach, used traditionally
in non-equilibrium statistical mechanics to describe steady-state or
transient phenomena. Simulations are used to verify all theoretical
predictions. We demonstrate the application of our methodology to
Chord, a recently introduced and very well-studied P2P network.
- 27.5.2005 Prof. Pekka Orponen:
Focused Local Search for Random 3-Satisfiability
Abstract:
An important heuristic in local search algorithms for
Satisfiability is focusing, i.e. restricting
the selection of flipped variables to those appearing
in presently unsatisfied clauses. We consider the behaviour
on large randomly generated 3-SAT instances of three
focused solution methods: the well-known WalkSAT algorithm,
the straightforward algorithm obtained by imposing
a focusing constraint on basic Metropolis dynamics,
and a less-known but very well performing method,
the Focused Record-to-Record Travel algorithm.
We observe that both WalkSAT and the Focused Metropolis
Search method are quite sensitive to the proper choice
of their "noise" and "temperature" parameters, and attempt
to estimate ideal values for these, such that the algorithms
exhibit generically linear solution times as close to the
satisfiability transition threshold alpha_c = 4.267 as possible.
With an appropriate choice of parameters, the linear time
regime of both algorithms seems to extend well into
clauses-to-variables ratios alpha > 4.2, which is much
further than has generally been assumed in the literature.
(Joint work with Sakari Seitz and Mikko Alava.)
- 3.6.2005 Dr. Tommi Junttila:
An Incremental and Layered Procedure for the Satisfiability of Linear
Arithmetic Logic
Abstract:
The problem of deciding the satisfiability of a quantifier-free
formula with respect to a background theory, also known as
Satisfiability Modulo Theories (SMT), is gaining increasing relevance
in verification: representation capabilities beyond propositional
logic allow for a natural modeling of real-world problems
(e.g., pipeline and RTL circuits verification, proof obligations in
software systems).
This talk presents a new decision procedure for the satisfiability of
Linear Arithmetic Logic (LAL), i.e. boolean combinations of propositional
variables and linear constraints over numerical variables.
The approach is based on the well known integration of a propositional SAT
procedure with theory deciders, enhanced in the following ways.
First, the procedure relies on an incremental solver for linear
arithmetic, that is able to exploit the fact that it is repeatedly
called to analyze sequences of increasingly large sets of
constraints. Reasoning in the theory of LA interacts with the boolean
top level by means of a stack-based interface, that enables the top
level to add constraints, set points of backtracking, and backjump,
without restarting the procedure from scratch at every call.
Sets of inconsistent constraints are found and used to drive
backjumping and learning at the boolean level, and theory atoms that
are consequences of the current partial assignment are inferred.
Second, the solver is layered: a satisfying assignment is
constructed by reasoning at different levels of abstractions (logic of
equality, real values, and integer solutions). Cheaper, more abstract
solvers are called first, and unsatisfiability at higher levels is
used to prune the search. In addition, theory reasoning is partitioned
in different clusters, and tightly integrated with boolean reasoning.
The effectiveness of the approach is demonstrated by means of a
thorough experimental evaluation: our approach is competitive with and
often superior to several state-of-the-art decision procedures.
The presentation is based on the TACAS 2005 article "An Incremental and
Layered Procedure for the Satisfiability of Linear Arithmetic Logic"
co-authored with Marco Bozzano, Roberto Bruttomesso, Alessandro Cimatti,
Peter van Rossum, Stephan Schulz, and Roberto Sebastiani.
- 10.6.2005 Dr. Keijo Heljanko:
Incremental and Complete Bounded Model Checking for Full PLTL
Abstract:
Bounded model checking is an efficient method for finding bugs in
system designs. The major drawback of the basic method is that it
cannot prove properties, only disprove them. Recently, some
progress has been made towards proving properties of LTL using
bounded model checking. We present an incremental and complete
bounded model checking method for the full linear temporal logic
with past (PLTL).
Compared to previous works, our method both improves and extends
current results in many ways: (i) our encoding is incremental,
resulting in improvements in performance, (ii) we can prove
non-existence of a counterexample at shallower depths in many
cases, and (iii) we support full PLTL. We have implemented our
method in the NuSMV2 model checker and report encouraging
experimental results.
(Joint work with Timo Latvala and Tommi Junttila.)
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Latest update: 07 June 2005.
Ilkka Niemelä
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