TCSA Day 2010
We are happy to invite you to the second annual

Theoretical Computer Science Amsterdam (TCSA) Day
which will take place on

Friday October 29, 2010
10:0017:00
at CWI, Science Park 123, 1098 XG Amsterdam.
Directions to CWI
in the Eulerroom (=room Z009); take the entrance to the left of the revolving door which is CWI's main entrance
The TCSA Day is intended to be an annual event, taking place in the Fall, to alternate with
the national NVTI Theory Day that takes place in Spring.
The event is organized jointly by CWI, UvA, and VU, and its aim is to foster cooperation among the various TCSreseachers in and around Amsterdam.
Last year's edition took place at the VU.
Its Steering Committee consists of Jan Bergstra (UvA), Harry Buhrman (CWI), and Wan Fokkink (VU).
The Organizing Committee are
Alban Ponse (UvA),
Femke van Raamsdonk (VU),
Leen Torenvliet (UvA),
and Ronald de Wolf (CWI).
The programme consists of talks by six researchers from CWI, UvA, and VU.
There is no need to register. Tea and coffee will be provided. Lunch is not organised, but there is a cafetaria in the CWI building and another one at the UvA building across the street.
Schedule
 09:30 coffee
 10:0010:45 Jan Rutten (CWI): Behavioural differential equations  a concrete framework for coinductive definitions
 10:4511:15 coffee
 11:1512:00 Pieter Adriaans (UvA): Facticity and meaningful information
 12:0012:45 Ulle Endriss (UvA): Complexity of judgment aggregation
 12:4514:00 lunch (not organised)
 14:0014:45 Harry Buhrman (CWI): Positionbased quantum cryptography: impossibility and constructions
 14:4515:30 Piet Rodenburg (UvA): Structuring partial algebras
 15:3016:00 tea
 16:0016:45 Jan Willem Klop (VU): Defining, transforming and comparing infinite streams
Abstracts
 10:0010:45 Jan Rutten (CWI): Behavioural differential equations  a concrete framework for coinductive definitions
Abstract:
Coinduction has come to play an ever more important role in
theoretical computer science, for the specification of and reasoning about
infinite data structures and, more generally,
automata with infinite behaviour.
In this talk, we shall focus on a recently introduced formalism
for coinductive definitions: behavioural differential equations,
with which one specifies behaviour in terms of initial outputs and
behavioural derivatives (next state operators). Our emphasis will be on
the elementary calculus of streams (infinite sequences), of which we shall
discuss the basic theory, developed in close analogy to
mathematical analysis.
As an application area, we will discuss a coinductive calculus
of periodic stream operators (joint work with Milad Niqui (CWI)).
Using this calculus, we will give a new
and transparent proof of Moessner's theorem using coinduction.
This theorem (from 1951  1952) gives a suprising construction
for the stream of powers n^k of the natural numbers
(such as 1,8,27,64, ... for k=3) out of the stream of
natural numbers by an alternating process of stream sampling
and taking partial sums.
 11:1512:00 Pieter Adriaans (UvA): Facticity and meaningful information
Abstract:
In my introductory course in learning theory I often make the remark that
the most informative television broadcast is pure white noise. Yet we do not
consider such a channel to be very interesting. So the notion of meaningful
information or 'interestingness' is not well covered by the standard
definitions. The wellknown formal definitions of the notion of information
(notably Kolmogorov complexity and Shannon Information) all have the counter
intuitive consequence that datasets with the highest entropy contain a
maximal amount of information. A number of authors have made useful
proposals in the past decennia to improve this situation. To name a few:
Sophistication (Koppel 1988, Anthunes, Fortnow 2007), Logical Depth (Bennet
1988), effective complexity (GellMann, Lloyd 2003), Meaningful Information
(Vitanyi 2003), Selfdissimilarity (Wolpert, Mcready 2007), Computational
Depth (Antunes et al. 2006). In a number of recent publications and lectures
I have proposed the notion of facticty (Adriaans 2009) to model meaningful
information. Basically it is the length of the model code in the context of
a variant of twopart code optimization in Kolmogorov complexity. In my
lecture I argue that this is the right definition and I analyze some of the
more puzzling consequences of this definition, notably the existence of
factic processes that create meaningful information.
 12:0012:45 Ulle Endriss (UvA): Complexity of judgment aggregation
Abstract:
Aggregating the judgments of a group of agents regarding a set of interdependent propositions, expressed in propositional logic, can lead to inconsistent outcomes. This paradox of judgment aggregation has recently sparked a good deal of interest in Legal Theory, Philosophy, Economics, and Computer Science. I will start with a short introduction to judgment aggregation and then report on our ongoing work aimed at better understanding this problem domain from a computational point of view. Specifically, we have analysed the computational complexity of several problems that naturally arise in the context of judgment aggregation: computing a collective judgment, strategic manipulation, and deciding whether a given class of aggregation problems can be guaranteed not to lead to paradoxical outcomes. I will also explain how this work fits into the broader research agenda on Computational Social Choice (COMSOC) currently being pursued at the ILLC.
This is joint work with Umberto Grandi and Daniele Porello.
 14:0014:45 Harry Buhrman (CWI): Positionbased quantum cryptography: impossibility and constructions
Abstract:
We study positionbased cryptography in the quantum setting. The aim is to use the geographical position of a party as its only credential. On the negative side, we show that if adversaries are allowed to share an arbitrarily large entangled quantum state, no secure positionverification is possible at all. That is, we show a generic attack that breaks any positionverification scheme of a very general form. On the positive side, we show that if adversaries do not share any entangled quantum state but can compute arbitrary quantum operations, secure positionverification is achievable. Jointly, these results suggest the interesting question whether secure positionverification is possible in case of a bounded amount of entanglement. Our positive result can be interpreted as resolving this question in the simplest case, where the bound is set to zero.
In models where secure positioning is achievable, it has a number of interesting applications. For example, it enables secure communication over an insecure channel without having any preshared key, with the guarantee that only a party at a specific location can learn the content of the conversation. More generally, we show that in settings where secure positionverification is achievable, other positionbased cryptographic schemes are possible as well, such as secure positionbased authentication and positionbased key agreement.
This is joint work with Nishanth Chandran, Serge Fehr, Ran Gelles, Vipul Goyal, Rafail Ostrovsky, and Christian Schaffner
 14:4515:30 Piet Rodenburg (UvA): Structuring partial algebras
Abstract:
In Typability in Partial Applicative Structures (with I. Bethke; to appear in the Journal of Logic, Language and Information; preliminary version at arxiv/0901.0188 under the title Typability in partial groupoids) conditions are established under which an algebra with a single, partial, binary operation can be given a simple type structure. In this talk, I will consider the problem of adding type structure to partial algebras with more operations.
 16:0016:45 Jan Willem Klop (VU): Defining, transforming and comparing infinite streams
Abstract:
We discuss two topics in infinite sequences (or streams).
The first topic is how to define streams in a uniform framework, that
we call Pure Stream Format (PSF). Specifically we concentrate on
proving welldefinedness of such specifications  an issue known in
computer science as productivity. We single out a restricted class of
specifications, still expressive enough to encompass automatic
streams, morphic streams, and more. For specifications in this
restricted format we can automatically decide productivity. The
second topic is concerned with an attempted classification of infinite
streams, by means of finite state transducers (FSTs), leading to a
notion of 'degree of stream' that is analogous to the wellknown
Turing degrees of unsolvability in mathematical logic. (In fact FSTs
correspond to context expressions in PSF.) We mention initial results
in the form of some elementary properties of this partial order of
stream degrees, concluding with our favourite conjecture about the
degree of the ThueMorse sequence.
This is joint work with Clemens Grabmayer (UU), Joerg Endrullis (VU) and Dimitri Hendriks (VU)
This page is maintained by Ronald de Wolf. Last update: Oct 5, 2010