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Computer Science Seminars and Short Courses — 2000/01

The Computer Science Group holds regular advanced and research level meetings, mainly during the two University teaching semesters. These meetings take the form of Research Seminars, which consist of one hour talks given by invited speakers or members of the Department on current research; Reading Groups which involve a small number of staff and research students studying an advanced book or research paper; and Lecture Courses which are usually delivered by a member of staff and pitched at advanced postgraduate level. Such lecture courses are also taken by staff and research students.

Semester 1

Seminar details

Applied categorical logic: Information systems specification in industry

Prof. Michael Johnson (Macquarie University - Division of Information and Comunication Sciences )
Thursday 21 September, 14:30 in Benett LT3

Category theory is sometimes viewed as even more esoteric than other branches of pure mathematics. Yet many of the basic concepts of category theory, are of great value in industrial consultancies.

This talk describes some of my consultancy work which is based on using categorical universal algebra in information system specification. Most of the talk should be comprehensible for people with no knowledge of category theory, nor of information system specification.

The Relative Complexity of Approximate Counting

Prof. Martin Dyer (Leeds University)
6 October, 14:30 in G3

Two natural classes of counting problems that are interreducible under approximation-preserving reductions are: (i) those that admit a particular kind of efficient approximation algorithm known as an FPRAS, and (ii) those that are complete for #P with respect to approximation-preserving reducibility. We describe and investigate not only these two classes but also a third class, of intermediate complexity, that is not known to be identical to (i) or (ii). The third class can be characterised as the hardest problems in a logically defined subclass of #P.

The Hospitals / Residents Problem with Ties

Dr. David Manlove (University of Glasgow, Department of Computer Science)
20 October, 14:30 in G3

In a number of countries, the annual process of assigning graduating medical students (also known as residents) to their first hospital appointments is accomplished by a centralised matching scheme. The allocation process involves constructing a stable matching, based on the preferences of residents over hospitals, and vice versa. In this talk, I will describe the classical algorithms that lie at the heart of these matching schemes. I will then outline some algorithmic consequences of permitting hospitals and residents to express ties in their preferences lists. In particular, I will present an algorithm for matching residents to hospitals when indifference is allowed. This work has been carried out jointly with Rob Irving and Sandy Scott, also of the University of Glasgow, as part of the EPSRC-funded Stable Matching Algorithms research project. The talk should be accessible to a general CS/Maths audience, including those with no prior knowledge of algorithmics.

Reasoning about Real-Time Programs Using Idle-Invariant Assertions

Dr. Ian Hays (Queensland University)
Wednesday 25 October, 14:30 in G3

We develop a set of laws for reasoning about real-time programs using assertions (preconditions and postconditions) in the style of Hoare. In the real-time context assertions may refer to the current time and to the value of external inputs, which are not under the direct control of the program and hence not guaranteed to be stable with respect to the passage of time (even if the program does not modify any of the variables under its control). Hence in order to reason about real-time programs, we make use of idle-invariant assertions: assertions that are invariant to just the passage of time.

Abstract Stone Duality - The Computational Side

Dr Paul Taylor (Queen Mary and Westfield College)
27 October, 14:30 in G3

Abstract Stone Duality is a new axiomatisation for topological spaces without arbitrary unions, intended to give them a direct computational basis. This talk will largely be concerned with the translation of these axioms into a programming language (PROLOG). There should be something for everybody - some topological ideas, some elementary category theory, some lambda calculus, some sequent calculus and some (standard) compiler techniques. The category theory and topology are needed to understand WHY I'm doing this, not HOW.

The equivalence of (strict) globular multiple categories and cubical multiple categories with connection

Prof. R.Brown (School of informatics, Mathematics division, University of Wales, Bangor)
3 November, 14:30 in G3

(Joint work with F A-A Al-Agl and R Steiner)

No previous knowledge of multiple categories will be assumed. We begin by reviewing the equivalence of 2-categories and double categories with connection (C B Spencer, 1977), showing how this gives a context for 2-dimensional algebra' . Then we explain the overall method of the recent extension to all dimensions. This extension allows for the description of a commutative cube' and for the construction of a monoidal closed structure in the globular case. The methods go back to the deep past of relative homotopy theory. There are current investigations (Goubault, Gaucher) on applications to concurrency.

Schemas as toposes

Dr Steve Vickers (Open University of Milton Keynes, Department of Pure Mathematics)
10 November, 14:30 in G3

One possible view of schemas such as those in Z is that they present logical theories: they declare some symbols and then set out some axioms expressed in terms of those symbols. This is not a trivial comparison, since there are features in Z - an obvious one is the type sytem - that do not translate directly into first-order logic. However, in the so-called "geometric" logic this kind of translation can be made to work despite the fact that the logic is still first-order. The trick is that the logic is infinitary and this allows some types to be characterized within the logic itself in a way that is impossible in ordinary classical logic. Geometric theories correspond to Grothendieck toposes, their so-called "classifying toposes", and this means that a schema (of the right form) can be viewed as presenting a topos. That might seem unwelcome news if you don't know what a topos is, but it does provide a clean link to a deep body of mathematics and in fact it illuminates the question of what toposes themselves are, particularly in their mysterious guise as "generalized topological spaces" - the topos provides a "space of implementations" of the schema. I shall also describe how the geometric logic can bear an observational interpretation that suggests it can have specificational aptness as well as mathematical elegance.

Programming modulo Alpha-conversion

Dr Andrew Pitts (Cambridge University, Computer Laboratory)
8 December, 14:30 in G3

This talk will describe work in progress on the design of a programming language for recursively defined functions on user-declared inductive datatypes involving binding constructors. The language has a novel type constructor for abstractions. Recursive definitions can use a form of pattern-matching on bound names of abstractions, while the type system ensures that only those definitions respecting alpha-conversion of bound names typecheck. The language design is semantically driven', in as much as it arises out of contemplating the model of variable-binding in terms of the Fraenkel-Mostowski permutation model of set theory given by Gabbay and Pitts in Proc. LICS '99, pp 214--224.

First-order projections and polynomial-space games

Pr Iain Stewart (Leicester)
Wednesday 13 December, 14:00 in Benett LT1

First-order projections are very restricted logical reductions between decision problems. They were developed by Neil Immerman in the 80's, and subsequently lots of problems have been proven complete for some complexity class via these reductions. In this talk I will show how the existence of a first-order projection from the problem TC (transitive closure) to some other problem $\Omega$ enables us to automatically deduce that a natural game problem, $\mathcal{LG}(\Omega)$, whose instances are labelled instances of $\Omega$, is complete for {\bf PSPACE} (via log-space reductions). Our analysis is strongly dependent upon the reduction from TC to $\Omega$ being a logical projection in that it fails should the reduction be, for example, a log-space reduction or a quantifier-free first-order translation. This seminar will be accessible to most in that I will only assume a nodding acquaintance with complexity theory and I will fully describe first-order projections and the associated games. (This is joint work with Argimiro Arratia, Universidad Simon Bolivar, Caracas.)

What is the Eckmann-Hilton argument?

Dr J.M.E. Hyland (Cambridge)
Wednesday 13 December, 15:00 in Benett LT1

The Eckmann-Hilton argument is generally regarded as the reason why the higher homotopy groups are abelian. I shall explain what it means in the context of algebraic structure on symmetric monoidal categories. This topic provides the foundations for a theory of wiring diagrams in computer science and is also a necessary step in the development of the higher dimensional linear algebra needed to provide an account of topological field theory. In the talk I shall concentrate on characterising some simple concrete categories of wiring diagrams. (This is an account of work done with John Power.)

The Generic Approximation Lemma

Dr Graham Hutton
Wednesday 13 December, 16:00 in Benett LT1

The approximation lemma is a simplification of the well-known take lemma, and is used to prove properties of programs that produce lists of values. In this talk I will show how the approximation lemma, unlike the take lemma, can naturally be generalised from lists to a large class of datatypes, and present a generic approximation lemma that is parametric in the datatype to which it applies.

Semester 2

Seminar details

New Tractable Classes From Old

Dr Richard Gault (Oxford University - Computing Laboratory)
26th January, 14:30 in G3

Many combinatorial problems can be naturally expressed as "Constraint Satisfation Problems" (CSPs). This class of problems is known to be NP-hard in general, but a number of restrictions of the general problem have been identified which ensure tractability. In this talk, we shall describe a framework, due to Jeavons, Cohen, et al., which enables a uniform description of these tractable subclasses. In addition, we will introduce a general method of combining two or more tractable subclasses, so as to generate new tractable subclasses. These new classes are genuinely novel, and have not previously been identified. In addition, our algorithm for solving them can be generalised to an algorithm for solving a much wider variety of CSPs than one might at first suppose.

Resource Semantics, Bunched Logic and Logic Programming Imperfect

Prof. David Pym (Queen Mary and Westfield College, Department of Computer Science)
16th February, 14:30 in G3

Starting from a basic model of resources, we derive BI, the logic of bunched implications, in which a multiplicative and an additive implication live side-by-side. Propositional BI may be seen to arise from an analysis of the proof-theoretic relationship between between conjunction and implication, and may be viewed ar a merging of intuitionistic logic and intuitionistic linear logic. Predicate BI includes, in addition to the usual additive quantifiers, multiplicative (or intensional) quantifiers, which arise from observing restrictions on structural rules at the level of terms. These restrictions allow the distinction between additive and multiplicative predication for each propositional connective. We discuss proof theory, forcing and categorical semantics for BI and describe a range of applications with particular emphasis on logic programming with BI, which comes along with a natural account of locality (sharing) and globality (non-sharing).

Imperfect information

Prof. Wilfrid Hodges (Queen Mary and Westfield College, School of Mathematics)
23rd February, 14:30 in G3

The logic of imperfect information has been around for some forty years, and makes regular appearances in the literature on natural language semantics. Its position in computer science is more tenuous, though there are hints that it or something like it ought to be useful for analysing imperfect communication of information between systems. My own interest in it is that I recently found a Tarski-style semantics for it (as opposed to the standard game-theoretic semantics given implicitly by Henkin in the 1950s), and this allowed us to prove some surprising facts about the logic. The talk will survey what is known, mentioning contributions of Henkin, Ehrenfeucht, Hintikka, Enderton,Walkoe, Blass, Gurevich, Vaananen, Hodges, Cameron, Caicedo, Krynicki, Sandu, Bradfield.

Periodicity of two-dimensional arrays

Dr J K Truss (University of Leeds)
9 March, 14:30 in G3

I propose a solution to a problem of Maurice Nivat. A {\em two-dimensional array} is a function $f$ from $Z^2$ to some 'alphabet' $\Sigma$. The array is said to be {\em periodic} if there is a non-zero vector ${\bf a}$ (necessarily with integer co-ordinates) such that $f({\bf x} + {\bf a}) = f({\bf x})$ for all ${\bf x}$ in $Z^2$. Nivat proposed a sufficient condition for the periodicity of $f$ in terms of the 'views' exhibited by $f$. Specifically, for positive integers $m$ and $n$, an $m \times n$ {\em view} is an $m \times n$ matrix which 'appears' somewhere in $f$ (in an obvious sense). Nivat's condition is that for some $m$ and $n$, there are at most $mn$ $m \times n$-views exhibited by $f$. I remark that this condition is definitely not {\em necessary} for periodicity, and I explain the two main ideas in a proof that it is sufficient. The first is that to establish periodicity, one tries to show that the whole array is determined by the list of views from a finite subarray, and the other is that to use induction, one should also consider restrictions of $f$ to more complicated subsets of $Z^2$ than just finite rectangles.

Priority Guards

Dr Iain Phillips (Imperial College)
16 March, 14:30 in G3

It has long been recognised that ordinary process algebra has difficulty dealing with actions of different priority, such as for instance an interrupt action of high priority. Various solutions have been proposed. We introduce a new approach, involving the addition of `priority guards'' to the summation operator of Milner's process calculus CCS. In our approach, priority is \emph{unstratified}, meaning that actions are not assigned fixed levels, so that the same action can have different priority depending where it appears in a program. An important feature is that, unlike in other unstratified accounts of priority in CCS (such as that of Camilleri and Winskel), we can treat inputs and outputs symmetrically.

Dr David Duke (University of Bath, Department of Mathematical Sciences)
23 March, 14:30 in G3

The design of an effective user interface often involves tacit understanding of users' perceptual and cognitive capabilities. However as interface designers explore new technologies such as speech and gesture, the complexity of interactions can confound craft-based intuitions. This talk explains the rationale behind an approach to modelling interaction in which formal methods are used to represent both the interface and a model of human cognitive resources. The aim is to use the expressive power of mathematical models to better understand the interaction between computational resources and cognitive constraints. Two recent directions opened up by this work will also be described. The first generalises the modelling approach to give a framework for "macro-theory", addressing interaction between agents across a range of abstraction levels. The second is a specific application of the modelling approach to understand issues in the visualisation of large-scale datasets.

Classifying the complexity of temporal constraints

Dr Andrei Krokhin (Oxford University - Computing Laboratory)
4 May

Reasoning about temporal constraints is an important task in many areas of computer science and elsewhere, such as scheduling, planning, technical diagnosis, molecular biology, and archeology. Several frameworks for formalizing this type of problems have been suggested; for instance, the {\em point algebra} (for expressing relations between time points), the {\em point-interval algebra} (for expressing relations between time points and intervals) and the famous {\em Allen's interval algebra} for expressing relations between time intervals. This algebra has proven to be useful and convenient and it has also become the kernel of some other formalisms. Due to computational hardness (the basic satisfiability problem in Allen's algebra is NP-complete), it is unlikely that efficient algorithms exists for reasoning in the full algebra. This computational difficulty has motivated the study of the complexity of fragments of the algebra, and the search for effective heuristics based on tractable fragments. In this talk we present a complete classification of complexity in Allen's algebra. This is a joint work with Peter Jeavons (Oxford University) and Peter Jonsson (Linkoeping University, Sweden)

An introduction to relation algebra and games

Dr Robin Hirsch (University College London, Department of Computer Science)
11 May, 14:30 in G3

Boolean algebra can be thought of as an algebraisation of fields of unary relations (sets). Stone's theorem states that every boolean algebra can be represented as a field of sets. Similarly, relation algebra is an algebraic treatment of fields of binary relations, but not every relation algebra can be represented as a field of sets. I'll show how to use games to characterise which relation algebras are representable.

I will then continue with one of two alternatives: (a) applications of these game-theoretic techniques to temporal constraints and planning, or (b) a connection between a probabilistic graph construction due to Erdos and a representation problem for relation algebras. If the audience is more interested in applications in computer science I will talk about (a) but if there is more interest in the mathematical aspects I will do (b).

Graph properties are often easier than you think

Prof. Etienne Grandjean (University of Caen, GREYC)
22 May

We are interested in some strong relationships between computational complexity and logical definability. We first recall how much the class NLIN (resp. DLIN), that is the class of problems recognizable in nondeterministic (resp. deterministic) linear time, is computationally and logically (resp. algebraically) robust. Note that NLIN contains most natural combinatorial NP problems. Secondly, we are interested by problems, regarded as sets of first-order finite structures (typically, sets of graphs G=(V,E), where E is a binary relation on the vertex domain V) and we study the class, denoted VertexNLIN, of the problems that are recognizable by RAM's in nondeterministic time O(n), where n is the size of the domain (e.g., for a graph, its number of vertices). Note that the computation time of such a (nondeterministic) RAM is too short to read all the input (graph) structure. We show that VertexNLIN is a robust class, from a logical (resp. computational) point of view. The interest of this class is enhanced by the fact (that we will show) that it contains a number of combinatorial graph properties, namely, connectivity, biconnectivity, hamiltonicity, nonplanarity, cubic subgraph, etc. Noticing that most of those properties are monotone, we will also present logical (resp. computational) characterizations of the subclass of monotone problems in VertexNLIN, which can be compared to similar characterizations of the larger class Monotone NP proved by Iain Stewart. Finally, we will prove that some graph properties are not in VertexNLIN and will present some structural properties of this class. (Common work with C. Lautemann, F. Olive and S. Ranaivoson).

Computing, is it Science?

Dr Paul Krause (Philips Research Laboratories)
25 May, 14:30 in G3

The theory of Computing goes back over three hundred years to Leibniz's design for a calculating machine. Since then it has exercised the minds of some of the world's finest mathematicians, and still continues to do so. But how well does the theory support the practice of Computing? If Computing is a science, then we ought to be able to use the theory to make predictions about the behaviour of our computing machines. Yet the modern world is littered with heroic failures where computing machines have shown, and continue to show, behaviour that appears to defy reason.

Computing fails as a science, because it lacks an experimental method. After three hundred years, we still do not have a clear consensus on how an hypothesis can be tested in the real world of developing complex software. The reasons for this will be discussed during this talk, and a way forward proposed. This last involves the use of probabilistic "causal" models. I will describe one such model and present results of its validation on a series of large-scale development projects at Philips.

Introduction to Parameterized Complexity

Dr Venkatesh Raman (The Institute of Mathematical Sciences, Chennai, India)
8 July, 14:30 in G4

Parameterized Complexity (developed by R. Downey and M. Fellows) is a recent promising approach to deal with computationally hard (typically NP-hard) problems. The idea is to identify a parameter k in the input which is likely to be small, and try to restrict the combinatorial exponential explosion to the parameter. More specifically, given a problem with input size n and a parameter k, the goal is to design, if possible, an algorithm whose running time is O(f(k) n^c) where c is a constant independent of k, and f is any (typically exponential) function of k. In this introduction, we will look at such algorithm design techniques illustrated through many examples. Some of the problems we will look at are: Given a graph G on n vertices, and a parameter k does it have a vertex cover (or a feedback vertex set, or a simple path or a simple cycle) on k vertices? Given a boolean formula in CNF form, is there an assignment to the variables satisfying at least k clauses? We will show that these problems have O(f(k)n^c) type algorithms and also look at some that are unlikely to have such an efficient algorithm.
 Author: Vincent Schmitt (V.Schmitt@mcs.le.ac.uk), T: 0116 252 3813.© University of Leicester 11th September 2000. Last modified: 23rd March 2005, 11:16:05CMS Web Maintainer. This document has been approved by the Head of Department.