2005
Special day on Mathematics of Cryptology
 
On January 21 we organize a joint meeting with the
RISC
seminar.
The lectures will take place at Room 201 (Huygens Building) of the
Lorentz Center in Leiden.
Program 
10:30−11:00  Coffee 
11:00−11:45 
Carles Padró (Barcelona)
Secret Sharing Schemes, Error Correcting Codes and Matroids
Abstract Error correcting codes and matroids have been widely used in the study of secret sharing schemes. This talk deals mainly with the connections between codes, matroids and a special class of secret sharing schemes, namely multiplicative linear secret sharing schemes (MLSSS). Such schemes are known to enable multiparty computation protocols secure against general (nonthreshold) adversaries. Two open problems related to the complexity of multiplicative linear secret sharing schemes will be considered. Hirt and Maurer proved that such a scheme can be costructed whenever the set of players is not the union of two unqualified subsets. Cramer, Damgard and Maurer proved that, in this case, a multiplicative linear secret sharing scheme can be efficiently constructed from any linear secret sharing scheme by increasing the complexity by a constant factor of 2. The first open problem we consider is to determine in which situations a multiplicative scheme can be obtained without increasing the complexity. We study this problem in an extremal case. Namely, to determine whether all selfdual vector space access structures admit an ideal MLSSS. By the aforementioned connection, this in fact constitutes an open problem about Matroid Theory, since it can be restated in terms of representability of identically selfdual matroids by selfdual codes. We introduce a new concept, the flatpartition, that provides a useful classification of identically selfdual matroids. Uniform identically selfdual matroids, which are known to be representable by selfdual codes, form one of the classes. We prove that this property also holds for the family of matroids that, in a natural way, is the next class in the above classification: the identically selfdual bipartite matroids. The second open problem deals with strongly multiplicative linear secret sharing schemes. As opposed to the case of multiplicative LSSSs, it is not known whether there is an efficient method to transform an LSSS into a strongly multiplicative LSSS for the same access structure with a polynomial increase of the complexity. We prove a property of strongly multiplicative LSSSs that could be useful in solving this problem. Namely, using a suitable generalization of the wellknown BerlekampWelch decoder, we show that all strongly multiplicative LSSSs enable efficient reconstruction of a shared secret in the presence of malicious faults. 
11:45−12:00  Coffee break 
12:00−12:45 
Salil Vadhan (Harvard)
Randomness Extractors and their Cryptographic Applications
Abstract: Over the past two decades, a rich body of work has developed around the problem of constructing randomness extractors  algorithms that extract highquality randomness (i.e. nearly uniform and independent bits) from lowquality randomness (i.e. sources of biased and correlated bits). Although some of the early results on randomness extraction came from the cryptography literature, most of the subsequent theory has been developed in the setting of computational complexity, where extractors have unified the study of a number of fundamental objects (such as pseudorandom generators, expander graphs, and listdecodable errorcorrecting codes). In the past few years, the relevance of extractors to cryptography has been rediscovered, with increasing variety of applications being found. In this talk, I will survey the basic theory of randomness extractors, give a sense of the current stateoftheart, and describe some of their cryptographic applications. 
12:45−13:45  Lunch 
13:45−14:30 
Rafi Ostrovsky (UCLA)
Survey on Private Information Retrieval
Abstract: Consider a setting where a user wishes to retrieve an item from a database, without letting the database administrator know which item is being retrieved. Of course, a trivial (but expensive) solution is for the user to request contents of the entire database, thus concealing from the database administrator which item is of interest to the user. Can this be done with less communication? Perhaps somewhat surprisingly, the answer is yes, under various assumptions and settings. In this talk, I'll survey much progress that has been achieved on this problem and point our some interesting connections to other problems in coding theory and several hardness results in cryptography. 
14:30−15:00  Tea break 
15:00−15:45 
Phong Nguyen (ENS Parijs)
From Euclid to LenstraLenstraLovasz: Revisiting Lattice Basis
Reduction
Abstract: Lattices are simple yet fascinating mathematical objects. Roughly speaking, they are linear deformations of the ndimensional grid Z^n. Lattices have many applications in mathematics and computer science. Of particular importance is lattice basis reduction, which is the problem of finding "nice" representations of lattices. For instance, lattice basis reduction is the most popular tool to attack publickey cryptosystems. In this talk, we will revisit lattice basis reduction, from Euclid's gcd algorithm to the celebrated LLL algorithm. We will also briefly discuss recent results. Curiously, it is possible to obtain a Euclidlike complexity for lattice basis reduction: in some sense, one can compute a reduced basis (without fast integer arithmetic) in essentially the same time as the elementary method to multiply integers. 
15:45−16:15  Tea break 
16:15−17:00 
Steven Galbraith
(London)
The Eta Pairing
Abstract: We introduce a new pairing on certain supersingular curves which is very closely related to the Tate pairing, but which has some implementation advantages. We interpret the results of Duursma and Lee in terms of this pairing and we describe a fast pairing on genus 2 curves in characteristic 2. 
17:00−18:00  Drinks 
February 4 
Special program in Utrecht on the occasion of Johnny Edwards' PhD defense
The afternoon talks are in room 018 at Kromme Nieuwegracht 66, number 11 on this map We can have lunch at "Trans 10", which is number 14 on the map 
10:30  PhD defense of Johnny Edwards in the Academiegebouw, number 1 on this map 
13:00−14:00 
Michael Stoll (Bremen), Proving nonexistence of rational
points on curves
Abstract. Let C be a curve of genus at least two over Q (or, more generally, over a number field K). An important problem is to decide whether C has any rational points. The first step in trying to solve this problem is to check if C has points everywhere locally (which can be done effectively). If this is the case, but C does not appear to have global points, there is the possibility that the absence of global points can be explained by the BrauerManin obstruction. It turns out that this is equivalent to the existence of a descent obstruction coming from some abelian covering of C. We will discuss how we can check for such an obstruction, focussing on genus 2 curves, and we will give experimental results obtained by Victor Flynn, Nils Bruin and myself. 
14:15−15:15 
Andreas Schweizer (KIAS Seoul), Construction of quadratic function
fields whose class groups have mrank 4.
Abstract. Fix a finite field F_{q} of odd characteristic and an odd integer m that is not divisible by the characteristic of F_{q}. We are interested in the mrank of the divisor class groups of quadratic extensions L of the rational function field F_{q}(T). First we show: If there is one L with mrank at least r, then there are infinitely many L with mrank at least r. Then for q = 1 mod 4 we explicitly construct an L with mrank at least 4. Similar constructions give slightly weaker results if q = 3 mod 4 and also for the ideal class group of the integral closure of F_{q}[T] in L. 
15:30−16:30 
Andreas Weiermann (Utrecht/Muenster),
Analytic combinatorics of the transfinite
Abstract: We consider a natural wellordered subclass H of Hardy's 1917 orders of infinity. Let H be the least set of functions from the non negative integers into the non negative integers such that: 1) the constant zero function belongs to H, 2) with f and g the function id^{f}+g belongs to H where id denotes the identity function. Let < be the ordering of eventual domination on H. A norm function is a function from H into the non negative integers such that for any non negative integer n there are only finitely many elements in H having norm below n. For a given norm N and a given f in H let c_{f}^{N}(n) be the number of elements g in H such that g<f and N(g)≤ n. We are going to classify various of these count functions. Partition functions appear as special cases of this construction. We use tools from Tauberian theory and generating function methodology. Moreover we indicate where the count functions can be used in logic. 
February 18  Leiden, room 312 (first talk), 405 (other talks). 
12:00−13:00 
Robin de Jong (Leiden),
Arakelov geometry and its applications to number
theory
Abstract. The purpose of this talk is to make popular the idea that Arakelov geometry can be used to obtain effective results in number theory. We discuss in this respect the existence of an efficient algorithm to compute the Ramanujan taufunction at a given prime number, as well as the effective Shafarevich conjecture trying to make quantitative the (proven) statement that given a number field K, a positive integer g and a set S of primes of K, there exist only finitely many K^algisomorphism classes of curves of genus g, defined over K and having good reduction outside S. The former topic is work in progress together with Edixhoven and Couveignes, the latter topic is probably still far out of reach, although recently strong results have been obtained by Heier in the function field case. In order to bring Arakelov theory into action, it is essential to obtain explicit representations of the various objects that occur in this theory. As an example we discuss an explicit formula for the ArakelovGreen function of a compact and connected Riemann surface of positive genus. 
14:00−14:45 
Eleonora Pellegrini,
Power bases for pure cubic fields
Abstract. In this talk we will consider the problem of the monogenicity of a number field, with a particular attention at cubic fields of the form Q(^{3}√m), where m is a cubefree positive integer. After giving a criterion of monogenicity, we will discuss the distribution of those m that give rise to monogenic pure cubic fields. 
15:00−15:45 
Clemens Fuchs (Leiden),
Polynomial generalisations of a problem of
Diophantus
Abstract. A Diophantine mtuple is a set of m positive integers with the property that theproduct of any two distinct elements plus one is a square of an integer. In my talk I will start with a survey on Diophantine mtuples and possible generalisations with an emphasis on variants of the problem for polynomials withinteger coefficients. E.g. I will discuss a recent joint result with A. Dujella and G. Walsh: we proved that there does not exist a set of more than 12 polynomials with integer coefficients, not all constant, and with the property that the product of any two of them plus a linear polynomial is a square of a polynomial with integer coefficients. 
16:00−16:45 
Capi Corrales Rodrigañez (Madrid),
On the unit group of an order in a nonsplit quaternion algebra
Abstract. I will speak on some joint work with E. Jespers, G. Leal and A. del Río, in which we give an algorithm to determine a finite set of generators of the unit group of an order in a nonsplit classical quaternion algebra H(K) over an imaginary quadratic extension K of the rationals. We then apply this method to obtain a presentation for the unit group of H(Z[(1+√7)/2]). As a consequence, we get a presentation for the orthogonal group SO_{3}(Z[(1+√7)/2]). These results provide the first examples of a characterization of the unit group of some group rings that have an epimorphic image that is an order in a noncommutative division algebra that is not a totally definite quaternion algebra. 
March 4  Universtiteit Gent, building S22 at Galglaan 2, lokaal 14. On the map of Gent this is between numbers 14 and 18, and on the Campus map it is building 4022 [further info]. 
13:3014:30  Joost van Hamel (Leuven), Abelianised Galois cohomology of reductive groups 
14:4515:45  Lenny Taelman (Groningen), On analytic unformization of abelian varieties and Anderson modules 
16:1517:15  Jan Schepers (Leuven), Introduction to motivic integration 
March 18  Groningen, room A901 at Broerstraat 9, next to the Academiegebouw (see the map) 
11:15−12:00 
Matthias Schuett (Hannover),
Extremal elliptic K3 fibrations
Abstract. In this talk we will consider elliptic K3 surfaces. After a brief introduction we will especially emphasize the extremal fibrations. With view to their classification and Frits Beukers' talk, we would like to discuss how many of them arise as pullback from rational elliptic surfaces by a base change of low degree. 
12:15−13:00 
Ronald van Luijk
(Berkeley),
K3 surfaces with Picard number one and infinitely many rational
points
Abstract. Not much is known about the arithmetic of K3 surfaces in general. Once the Picard number, which is the rank of the NeronSeveri group, is high enough, more structure is known and more can be said. But still we don't know of a single K3 surface whose set of rational points has been proved to be neither empty, nor Zariski dense. Also, until recently, not even a single K3 surface was known with NeronSeveri rank 1 and infinitely many rational points. We will give an explicit example of such a surface over Q, where even the Picard number over the algebraic closure is equal to 1. This solves an old problem, that has been attributed to Mumford. The method used has been extended by Remke Kloosterman to find elliptic K3 surfaces of rank 15. 
13:45−14:30 
Frits Beukers (Utrecht),
Computation of extremal elliptic K3 fibrations
Abstract. In a joint effort with H. Montanus we computed all extremal semistable elliptic K3 fibrations over P^{1}. Although the subject matter is geometrical, this talk will be computational. We focus our attention on the determination of 188 plane 'dessins d'enfant' and their associated Belyi maps. 
14:45−15:30 
Jozef Steenbrink (Nijmegen),
The billiard problem on an ellipse
Abstract. I will show how one may visualize the solution to the billiard problem on an ellipse and how this gives rise to a nice deformation of a double hyperbola. It is a demonstration of the use of the CABRIgeometry program on academic level (I hope). 
16:15  PhD defense of Remke Kloosterman in the Academiegebouw 
April 1  Utrecht, room K11 
11:00−12:00 
Roelof Bruggeman (Utrecht),
Period functions and Maass cusp forms
Abstract. For holomorphic modular cusp forms there is a well established relation with period polynomials. In the case of modular Maass cusp forms, Lewis and Zagier have studied the associated period functions. I'll speak about ongoing work of Lewis, Zagier and me on these period functions and their cohomological interpretation. 
12:15−13:15 
Yiannis Petridis (Bonn),
Modular symbols and spectral theory
Abstract. A conjecture of Goldfeld about periods of elliptic curves implies a weak version of ABC. On the other hand this conjecture is equivalent with the modular symbol conjecture about their relative growth. The modular symbols encode geometric information about the cohomology and the fundamental group of the modular curves. Through trace typeidentities we can relate modular symbols and the spectral theory of the Laplace operator on the modular curve. The objects to study are: families of automorphic forms, Eisenstein series twisted by modular symbols and Selbergzeta functions. The technique is perturbation theory of operators. The result is the distribution of the (normalized) modular symbols: a Gaussian law. 
14:15−15:15  Byoung Ki Seo (Utrecht),
Asymptotic behaviors of the first return time of
translations on a torus
Abstract. We investigate asymptotic behaviors of the first return time of translations on a torus using Diophantine approximations. It is known that the first return time to an element of the equipartition equals to the 1 over the size of the element asymptotically for almost but not all irrational translation on a 1 dimensional unit interval. We expand the results to the case for an irrational translation on a multidimensional torus. 
15:30−16:30 
Gunther Cornelissen (Utrecht),
Complexity of the rational numbers and conjectures about elliptic
curves
Abstract. We don't know whether or not there is an algorithm to decide whether a diophantine equation has a rational solution or not ("Hilbert's tenth problem for Q", HTP(Q)), but Julia Robinson has proven in 1949 that one cannot decide the truth of more general statements about the rationals. I will discuss conjectures about elliptic curves that allow us to improve upon Robinson's result and  in a sense that I will make precise  closer to a solution of HTP(Q). The keyword is "Zsigmondy type theorems for elliptic divisibility sequences with extra conditions". I will present some heuristics to support the conjectures. 
April 15 
Special day on Lambda ringsNijmegen, room CZ N7 (=N1004 on the first floor of building N1)On the map look for the red dot with green arrow "oudbouw". Enter there and go up two floors. For parking you might get lucky at the bottom of the map near "INGANG VANAF BRAKKESTEIN" on the Driehuizerweg, which is easier to spot on this map Speakers: Jim Borger (Max Planck), Frans Clauwens (Nijmegen), Bart de Smit (Leiden). 
12:00−13:00  Jim Borger (Max Planck), Lambda rings for beginners part I 
14:00−15:00  Jim Borger (Max Planck), Lambda rings for beginners part II 
15:15−16:15  Frans Clauwens (Nijmegen), Natural operations on lambda rings 
16:30−17:30  Bart de Smit (Leiden),
Integral lambda ring structure on finite étale Qagebras
Abstract. For a lambda ring K which is finite étale as a Qalgebra we determine whether K has a lambda subring R of finite rank over Z so that K=R⊗Q. We also determine the maximal such R if it exists. This is joint work with Jim Borger. 
May 13  Leiden, room 412 
13:30−14:15 
Joost Batenburg (Leiden/CWI),
Small, smaller, smallest. Steps toward the atomic resolution
microscope.
Abstract. Building an electron microscope that can view samples at atomic resolution is a longstanding goal in the microscopy community. It has recently become possible to acquire images of projected crystalline structures in which separate atom columns can be resolved. By themselves, these images do not provide sufficient information to determine the positions and types of individual atoms inside the crystal. However, when projections from several viewpoints are combined, it is possible to make a full 3D reconstruction of the crystal by performing a tomographic inversion procedure. Because the number of measured projections is typically very small, methods from continuous tomography, which are used in medical imaging, cannot be used for this application. The young field of discrete tomography is suited particularly well for computing reconstructions from few projections. Discrete tomography is an interdisciplinary field, with links to number theory, combinatorics, operations research and coding theory. In this talk I will describe how discrete tomography can be used to compute atomic resolution 3D reconstructions of crystals and discuss a variety of problems that still need to be solved. 
14:30−15:15 
Oliver Lorscheid (Utrecht),
Completeness and compactness for varieties over local fields
[math.AG/0410346]
Abstract. For a complex variety it is well known that it is complete if and only if the rational points form a compact space in the complex topology. The only essential property of the complex numbers for this statement to hold is their locally compactness. One finds the following generalisation: let K be a local field and X a variety over K, then X is complete if and only if for every finite field extension L of K, X(L) is compact in its strong topology. 
15:30−17:00 
PreJournées program Three PhD students from Leiden present their proposed talks for the Journées Arithmétiques
Willem Jan Palenstijn,
Computing nearprimitive root densities
Bas Jansen,
Mersenne primes and Lehmer's observation
Reinier Bröker,
Class invariants in a nonarchimedean setting

September 2 
Special program around PhD defense of
Gabor Wiese.
Location of the talks: Spectrumzaal of Studentencentrum Plexus, Kaiserstraat 25, Leiden ( directions) 
10:00−10:45 
Gabor Wiese (Leiden),
Modular Forms of Weight One Over Finite Fields.
Abstract. The talk having the same title as my thesis to be defended in the afternoon will start by presenting the main motivation for my research, namely (some of) the number theoretic information really and conjecturally provided by modular forms. Next, there will be a short overview over the thesis. Finally, I will sketch the proof of one of the results. 
11:00−11:45 
Loïc Merel (Paris),
The formula relating modular symbols to Lvalues.
Abstract. There are well known formulas relating values of Lfunctions of modular forms to modular symbols. These formulas enable to construct padic Lfunctions etc. Modular symbols contain a finite generating set consisting of the socalled Manin symbols. I will describe how one can express a Manin symbol at level N in terms of Lvalues obtained by twisting modular forms of level N by characters of level dividing N and of a few local invariants. Here is an elementary corollary of this formula : the regular representation of Gal(Q(μ_{N})/Q), where Q(μ_{N}) is the field generated by a primitive Nth root of unity, does not occur in the group of Q(μ_{N})rational points of an elliptic curve E over Q of conductor N. 
14:15−15:00  Thesis defense of Gabor Wiese at the Academiegebouw, Rapenburg 73 in Leiden. 
Special day on the ABCconjecture, September 9 2005
This is the kickoff meeting of an NWO sponsored "Leraar in Onderzoek" project that will help Kennislink to take ABC to the masses.
September 9  Leiden, room 412. 
11:1512:00  Frits Beukers (Utrecht), Introduction to the ABC conjecture [PDF] 
12:1512:45  Jaap Top (Groningen), Finding good ABC triples, part I; notes in Dutch (PDF) 
12:4514:00  Lunch 
14:0014:30  Johan Bosman (Leiden), Finding good ABC triples, part II; notes in Dutch (PDF) 
14:4515:30  Hendrik Lenstra (Leiden), Granville's
theorem;
notes in Dutch (PDF)
Abstract. Barry Mazur defined the `power' of a number to be the logarithm of the number to the base its radical. For example, every perfect square has power at least 2. How many integers up to a large bound have power at least a given number? This question is answered by Granville's theorem. It is of importance both in understanding why the ABCconjecture has a chance of being true, and in analyzing an algorithm for enumerating ABCtriples. 
15:4516:15  Willem Jan Palenstijn (Leiden), Enumerating
ABC triples;
notes in Dutch (PDF)
Abstract. An ABC triple is a triple of coprime positive integers a, b, c with a + b = c and c larger than the radical of abc. In this talk we present an algorithm that enumerates all ABC triples with c smaller than a given upper bound N with a runtime essentially linear in N. 
16:3017:00  Carl Koppeschaar (Kennislink), Reken mee met ABC [PPT] 
September 23 
Utrecht room K11 Joint session with Intercity Arithmetic Geometry 
11:0011:45 
Bas Edixhoven,
Introduction to Serre's conjecture [PDF]
Abstract. The conjecture will be stated, and put in its historical context and in the wider context of the Langlands program. Serre's level and weight of a 2dimensional mod p Galois representation will be defined. Khare's result will be stated. An overview will be given of what will be treated in the seminar. 
12:0012:45 
Johan Bosman,
Galois representations associated to modular forms [PDF]
Abstract. Modular forms, Hecke operators and eigenforms will be defined. The existence of Galois representations associated to eigenforms will be stated. The construction of these representations will be postponed until later, in the more general case of Hilbert modular forms. Something about weight 2 being easier and about weight 1 being special can be said. 
13:3015:15 
Gunther Cornelissen,
Remarks on a conjecture of Fontaine and Mazur [PS]
Abstract. The following statement will be explained and proved: Let K be a number field, and S a finite set of places of K. Let K_{S} be a maximal algebraic extension of K in which all places in S are completely split. Let X be a smooth irreducible quasiprojective scheme over K, such that for every v in S the set X(K_{v}) is nonempty. Then X(K_{S}) is Zariski dense in X. 
October 7 
Leiden, room 312.
Joint session with Intercity Arithmetic Geometry 
12:1513:00 
Sander Dahmen,
Lower bounds for discriminants,
Abstract. Lower bounds for discriminants of number fields will be roved, in terms of their degree only; see Odlyzko 
14:0014:45 
Frits Beukers,
Upper bounds for discriminants,
Abstract. Let p be prime. Suppose that r is an irreducible representation of the absolute Galois group of Q on a 2dimensional vector space over an algebraic closure of F_{p}, with p in {2,3}. The image of such a representation is finite, hence determines a number field K that is Galois over Q. An upper bound for the discriminant of K will be proved that contradicts the lower bound of the previous lecture in the case that p is in {2,3} and r is unramified outside p. The audience will draw the logical conclusion. For p=2 this result is due to Tate, and for p=3 it is due to Serre. 
15:0015:45 
Bas Edixhoven,
Overview of Khare's proof
Abstract. An overview will be given of Khare's proof of Serre's conjecture in level one. Those who will not attend the rest of the seminar will have an idea of Khare's proof, and it is hoped that those who will attend the rest of the seminar will now be sufficiently motivated to digest the more technical parts that are to come. 
16:0016:45 
Johan de Jong,
Kloosterman lecture:
Rational points and rational connectivity
Abstract. Recently, Graber, Harris and Starr proved that any family of rationally connected projective varieties over a smooth curve has a section. A complex projective variety (or manifold) M is rationally connected when every two points in M lie on a rational curve in M. In this lecture we will explain a generalization of this result, joint with Jason Starr, to the case when the base of the family is a surface. In the other three lectures (October 14, November 11 and November 25) we will explain the idea behind the proof of the theorem, the analogy with Tsen's theorem, the connection with weak approximation and the connection with the periodindex problem for Brauer groups. 
October 21  Delft, Snijderszaal (1e verdieping laagbouw EWI), Mekelweg 4.  
11:0011:45 
Klaas Pieter Hart (Delft),
Ultrafilters and combinatorics
Abstract. In this talk I describe how ultrafilters may be used to prove combinatorial theorems about subsets of the set of natural numbers. For example, an ultrafilter helps in making various choices in a standard proof of Ramsey's theorem. For more intricate applications one needs to bring in some algebra. One can extend ordinary addition to the whole set of ultrafilters and thus obtain a compact semitopological semigroup. An idempotent in this semigroup is instrumental in proving Hindman's finitesum theorem and a special idempotent can be used to prove Van der Waerden's theorem on arithmetic progressions.  
12:0013:00 
Valerie Berthe (Montpellier),
Some applications of numeration systems
Abstract. The aim of this talk is to survey several applications of numeration systems, first in discrete geometry, and second, in cryptology. We first detail some natural connections between the study of discrete lines (and more generally discrete hyperplanes), and betaexpansions and tiling theory. For the latter application to cryptology, we then focus on some redundant numeration systems in mixed bases.  
14:0015:00 
Tom Ward (East Anglia), Heights and highly effective
Zsigmondy theorems
Abstract. Elliptic divisibility sequences are known to eventually have primitive divisors (indeed, the primitive part is very large). This talk will describe some families of elliptic divisibility sequences for which effective bounds can be given. For example, every term beyond the fourth term in the Somos4 sequence 1,1,1,1,2,3,7,23,... is shown to have a primitive divisor.  
15:1516:15 
JanHendrik Evertse (Leiden),
Linear equations with unknowns from a multiplicative group
Abstract. Let G be a finitely generated multiplicative group contained in the complex numbers, and let a, b be nonzero complex numbers. In 1960, Lang (building further on work of Siegel and Mahler), proved that the equation
has at most finitely many solutions x, y in G.
 
16:3017:15 
Robbert Fokkink (Delft),
Minimal sets and Euclidean rings
Abstract. Recently J.P. Cerri settled some conjectures on the euclidean minimum of a number field, using results from topological dynamics. This may seem like a remarkable link between two unrelated fields, but it really is not. 
November 4  Groningen, Room WSN 31, WSNbuilding 
11:3012:30 
Lenny Taelman,
An introduction to Drinfeld's shortest paper
Abstract. The paper referred to in the title is called "Commutative subrings of certain noncommutative rings" (Funkcional. Anal. i Prilozen. 11 (1977), no. 1, 1114). We discuss its results and applications to number theory and differential equations. 
13:1514:15 
Marius van der Put,
A localglobal problem for differential equations
Abstract. Consider a linear differential operator L:=a_{n}(d/dz)^{n} + ... + a_{1}(d/dz) + a_{0}. The question studied in the lecture is: if the inhomogeneous equation L(y)=f has a formal solution everywhere locally, does it follow that a global solution exists? 
14:3015:00 
Khuong An Nguyen,
dsolvable linear differential equations
Abstract. A linear differential equation over a differential field K is called dsolvable if all its solutions are in a tower K⊂K_{1}⊂K_{n} of differential fields, in which each K_{i+1}/K_{i} is either a finite extension or an extension obtained by adjoining to K_{i} all solutions and their (higher) derivatives of some linear diff. eq. over K_{i} of degree at most d. We discuss this notion and give examples. 
15:0015:45 
Steve Meagher,
Forms of algebraic curves
Abstract. Consider the Fermat curve X^{4} + Y^{4} + Z^{4} = 0; this curve has 96 automorphisms, and thus many `forms' (i.e., curves defined over the same ground field, which are not isomorphic to the given one but which become isomorphic over an extension field). We compute these forms and give a formula for their number of points over a finite field. 
16:0017:00 
Heide GluesingLuerssen,
A survey on convolution codes
Abstract. Convolutional codes are (free) submodules of F[z]^{n}, for F a finite field. They can be interpreted as encoding arbitrarily long sequences of message blocks into sequences of codeword blocks. As is the case for classical block codes, a certain weight function is needed for describing and investigating the errorcorrecting properties of the code. In this talk I will explain the basic notions for convolutional codes. Thereafter, I will sketch some current research projects. 
December 16  Leiden, Room 412 
11:3012:30 
Sven Verdoolaege
(LIACS, Leiden),
Enumerating parametric convex integer sets and their projections
Abstract. Given a convex set of integer tuples defined by linear inequalities over a fixed number of variables, where some of the variables are considered as parameters, we consider two different ways of representing the number of elements in the set in terms of the parameters. The first is an explicit function which generalizes Ehrhart quasipolynomials. The second is its corresponding generating function and generalizes the classical Ehrhart series. Both can be computed in polynomial time based on Barvinok's unimodular decomposition of cones. Furthermore, we can convert between the two representations in polynomial time. Finally, we present some ideas on how to handle projections of convex integer sets. 
13:3014:15  Willemien Ekkelkamp (CWI/Leiden),
A variation of the MPQS factoring algorithm: analysis and
experiments
Abstract. One of the methods used for factoring is the multiple polynomial quadratic sieve factoring algorithm. In this talk I will present a variation of this algorithm and some experimental results. Further, we will take a look at the (theoretical) ratio of the different types of relations coming out of the sieve. This ratio will be used in a simulation program for estimating the time needed for factoring a number. 
14:3015:15  Jeanine Daems (Leiden),
A historical introduction to mathematical crystallography
Abstract. What is a crystallographic group? How many crystallographic groups are there? The first part of Hilbert's 18th problem deals with crystallographic groups. In 1910 Bieberbach solved this problem by proving that the number of crystallographic groups in each dimension is only finite. In this talk I will discuss Bieberbach's proof and a modern proof of the same theorem. In dimensions 2, 3 and 4 the exact number of crystallographic groups is known, I will talk about the methods used to find them. 
15:3016:30  Peter Stevenhagen (Leiden),
Algorithmic class field theory
Abstract. Class field theory `describes' the abelian extensions of a number field, but does not readily provide generators for these extensions. Finding a `natural' set of generators is a Hilbert problem that is still largely unsolved. We will discuss to which extent the theory furnishes algorithms to generate class fields, and how this can be applied in algorithmic practice. 