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Koen Thas

Publications and source records attributed to Koen Thas.

33 records · Page 2Linked to original sources
arXiv2016

Automorphisms of Deitmar schemes, I. Functoriality and Trees

In a recent paper [3], the authors introduced a map $\mathcal{F}$ which associates a Deitmar scheme (which is defined over the field with one element, denoted by $\mathbb{F}_1$) with any given graph $Γ$. By base extension, a scheme $\mathcal{X}_k = \mathcal{F}(Γ) \otimes_{\mathbb{F}_1} k$ over any field $k$ arises. In the present paper, we will show that all these mappings are functors, and we will use this fact to study automorphism groups of the schemes $\mathcal{X}_k$. Several automorphism groups are considered: combinatorial, topological, and scheme-theoretic groups, and also groups induced by automorphisms of the ambient projective space. When $Γ$ is a finite tree, we will give a precise description of the combinatorial and projective groups, amongst other results.

math.AG↗
arXiv2015

Counting points and acquiring flesh

This set of notes is based on a lecture I gave at "50 years of Finite Geometry | A conference on the occasion of Jef Thas's 70th birthday," in November 2014. It consists essentially of three parts: in a first part, I introduce some ideas which are based in the combinatorial theory underlying $\mathbb{F}_1$, the field with one element. In a second part, I describe, in a nutshell, the fundamental scheme theory over $\mathbb{F}_1$ which was designed by Deitmar. The last part focuses on zeta functions of Deitmar schemes, and also presents more recent work done in this area.

math.AG↗
arXiv2015

Isospectral drums and simple groups

Virtually every known pair of isospectral but nonisometric manifolds - with as most famous members isospectral bounded $\mathbb{R}$-planar domains which makes one "not hear the shape of a drum" [13] - arise from the (group theoretical) Gassman-Sunada method. Moreover, all the known $\mathbb{R}$-planar examples (so counter examples to Kac's question) are constructed through a famous specialization of this method, called transplantation. We first describe a number of very general classes of length equivalent manifolds, with as particular cases isospectral manifolds, in each of the constructions starting from a given example that arises itself from the Gassman-Sunada method. The constructions include the examples arising from the transplantation technique (and thus in particular the planar examples). To that end, we introduce four properties - called FF, MAX, PAIR and INV - inspired by natural physical properties (which rule out trivial constructions), that are satisfied for each of the known planar examples. Vice versa, we show that length equivalent manifolds with FF, MAX, PAIR and INV which arise from the Gassman-Sunada method, must fall under one of our prior constructions, thus describing a precise classification of these objects. Due to the nature of our constructions and properties, a deep connection with finite simple groups occurs which seems, perhaps, rather surprising in the context of this paper. On the other hand, our properties define physically irreducible pairs of length equivalent manifolds - "atoms" of general pairs of length equivalent manifolds, in that such a general pair of manifolds is patched up out of irreducible pairs - and that is precisely what simple groups are for general groups.

math.GR↗
arXiv2015

Deitmar schemes, graphs and zeta functions

In [19] it was explained how one can naturally associate a Deitmar scheme (which is a scheme defined over the field with one element, $\mathbb{F}_1$) to a so-called "loose graph" (which is a generalization of a graph). Several properties of the Deitmar scheme can be proven easily from the combinatorics of the (loose) graph, and known realizations of objects over $\mathbb{F}_1$ such as combinatorial $\mathbb{F}_1$-projective and $\mathbb{F}_1$-affine spaces exactly depict the loose graph which corresponds to the associated Deitmar scheme. In this paper, we first modify the construction of loc. cit., and show that Deitmar schemes which are defined by finite trees (with possible end points) are "defined over $\mathbb{F}_1$" in Kurokawa's sense; we then derive a precise formula for the Kurokawa zeta function for such schemes (and so also for the counting polynomial of all associated $\mathbb{F}_q$-schemes). As a corollary, we find a zeta function for all such trees which contains information such as the number of inner points and the spectrum of degrees, and which is thus very different than Ihara's zeta function (which is trivial in this case). Using a process called "surgery," we show that one can determine the zeta function of a general loose graph and its associated { Deitmar, Grothendieck }-schemes in a number of steps, eventually reducing the calculation essentially to trees. We study a number of classes of examples of loose graphs, and introduce the Grothendieck ring of $\mathbb{F}_1$-schemes along the way in order to perform the calculations. Finally, we compare the new zeta function to Ihara's zeta function for graphs in a number of examples, and include a computer program for performing more tedious calculations.

math.AG↗
arXiv2014

On the mathematical foundations of mutually unbiased bases

In order to describe the right setting to handle Zauner's conjecture on mutually unbiased bases (MUBs) (saying that in $\mathbb{C}^d$, a set of MUBs of the theoretical maximal size $d + 1$ exists only if $d$ is a prime power), we pose some fundamental questions which naturally arise. Some of these questions have important consequences for the construction theory of (new) sets of maximal MUBs.

quant-ph↗
arXiv2014

Unextendible mutually unbiased bases (after Mandayam, Bandyopadhyay, Grassl and Wootters)

We consider questions posed in a recent paper of Mandayam, Bandyopadhyay, Grassl and Wootters [10] on the nature of "unextendible mutually unbiased bases." We describe a conceptual framework to study these questions, using a connection proved by the author in [19] between the set of nonidentity generalized Pauli operators on the Hilbert space of $N$ $d$-level quantum systems, $d$ a prime, and the geometry of non-degenerate alternating bilinear forms of rank $N$ over finite fields $\mathbb{F}_d$. We then supply alternative and short proofs of results obtained in [10], as well as new general bounds for the problems considered in loc. cit. In this setting, we also solve Conjecture 1 of [10], and speculate on variations of this conjecture.

quant-ph↗
arXiv2014

Hyperfield extensions, characteristic one and the Connes-Consani plane connection

Inspired by a recent paper of Alain Connes and Catherina Consani which connects the geometric theory surrounding the elusive field with one element to sharply transitive group actions on finite and infinite projective spaces ("Singer actions"), we consider several fudamental problems and conjectures about Singer actions. Among other results, we show that virtually all infinite abelian groups and all (possibly infinitely generated) free groups act as Singer groups on certain projective planes, as a corollary of a general criterion. We investigate for which fields $\mathbb{F}$ the plane $\mathbf{P}^2(\mathbb{F}) = \mathbf{PG}(2,\mathbb{F})$ (and more generally the space $\mathbf{P}^n(\mathbb{F}) = \mathbf{PG}(n,\mathbb{F})$) admits a Singer group, and show, e.g., that for any prime $p$ and any positive integer $n > 1$, $\mathbf{PG}(n,\overline{\mathbb{F}_p})$ cannot admit Singer groups. One of the main results in characteristic $0$, also as a corollary of a criterion which applies to many other fields, is that $\mathbf{PG}(m,\mathbb{R})$ with $m \ne 0$ a positive even integer, cannot admit Singer groups.

math.GR↗
arXiv2014

A criterion concerning Singer groups of generalized quadrangles, and construction of uniform lattices in $\widetilde{\mathbf{C}_2}$-buildings

We describe a simple criterion to construct Singer groups of Payne-derived generalized quadrangles, yielding, as a corollary, a classification of Singer groups of the classical Payne-derived quadrangles in any characteristic. This generalizes recent constructions of Singer groups of these quadrangles that were presented in a paper by Bamberg and Giudici. In the linear case, and several other cases, our classification is complete. Contrary to what seemed to be a common belief, we show that for the classical Payne-derived quadrangles, the number of different Singer groups is extremely large, and even bounded below by an exponential function of the order of the ground field. Our results have direct applications to the theory of $\widetilde{\mathbf{C}_2}$-buildings, which are explained at the end of the paper.

math.GR↗
arXiv2014

An obstruction relating locally finite polygons to translation quadrangles

One of the most fundamental open problems in Incidence Geometry, posed by Tits in the 1960s, asks for the existence of so-called "locally finite generalized polygons" | that is, generalized polygons with "mixed parameters" (one being finite and the other not). In a more specialized context, another long-standing problem (from the 1990s) is as to whether the endomorphism ring of any translation generalized quadrangle is a skew field (the answer of which is known in the finite case). (The analogous problem for projective planes, and its positive solution, the "Bruck-Bose construction," lies at the very base of the whole theory of translation planes.) In this short note, we introduce a category, representing certain very specific embeddings of generalized polygons, which surprisingly controls the solution of both (apparently entirely unrelated) problems.

math.CO↗
arXiv2014

The combinatorial-motivic nature of $\mathbb{F}_1$-schemes

We review Deitmar's theory of monoidal schemes to start with, and have a detailed look at the standard examples. It is explained how one can combinatorially study such schemes through a generalization of graph theory. In a more general setting we then introduce the author's version of $\mathbb{F}_1$-schemes (called $Υ$-schemes here), after which we study Grothendieck's motives in some detail in order to pass to "absolute motives". Throughout several considerations about absolute zeta functions are written. In a final part of the chapter, we describe the approach of Connes and Consani to understand the ad`ele class space through hyperring extension theory, in which a marvelous connection is revealed with certain group actions on projective spaces, and brand new results in the latter context are described. Many questions are posed, conjectures are stated and speculations are made.

math.AG↗
arXiv2014

The Weyl functor - Introduction to Absolute Arithmetic

Starting from an ancient observation of Tits concerning the interpretation of symmetric groups as Chevalley groups over a (non-existing) field having only one element, we describe combinatorial geometry over this field, as well as Linear Algebra. We arrive at an "absolute mantra" which is one of the basic principles of the present book.

math.GR↗
arXiv2014

Classification of STGQs, I

We describe new classification results in the theory of generalized quadrangles (= Tits-buildings of rank $2$ and type $\mathbb{B}_2$), more precisely in the (large) sub theory of skew translation generalized quadrangles ("STGQs"). Some of these involve, and solve, long-standing open problems.

math.CO↗
arXiv2014

Central aspects of skew translation quadrangles, I

Except for the Hermitian buildings $\mathcal{H}(4,q^2)$, up to a combination of duality, translation duality or Payne integration, every known finite building of type $\mathbb{B}_2$ satisfies a set of general synthetic properties, usually put together in the term "skew translation generalized quadrangle" (STGQ). In this series of papers, we classify finite skew translation generalized quadrangles. In the first installment of the series, as corollaries of the machinery we develop in the present paper, (a) we obtain the surprising result that any skew translation quadrangle of odd order $(s,s)$ is a symplectic quadrangle; (b) we determine all skew translation quadrangles with distinct elation groups (a problem posed by Payne in a less general setting); (c) we develop a structure theory for root-elations of skew translation quadrangles which will also be used in further parts, and which essentially tells us that a very general class of skew translation quadrangles admits the theoretical maximal number of root-elations for each member, and hence all members are "central" (the main property needed to control STGQs, as which will be shown throughout); (d) we solve the Main Parameter Conjecture for a class of STGQs containing the class of the previous item, and which conjecturally coincides with the class of all STGQs.

math.CO↗
arXiv2011

The 2-Transitive Transplantable Isospectral Drums

For Riemannian manifolds there are several examples which are isospectral but not isometric, see e.g. J. Milnor [Proc. Nat. Acad. Sci. USA 51 (1964), 542]; in the present paper, we investigate pairs of domains in ${\mathbb R}^2$ which are isospectral but not congruent. All known such counter examples to M. Kac's famous question can be constructed by a certain tiling method ("transplantability") using special linear operator groups which act 2-transitively on certain associated modules. In this paper we prove that if any operator group acts 2-transitively on the associated module, no new counter examples can occur. In fact, the main result is a corollary of a result on Schreier coset graphs of 2-transitive groups.

math-ph↗