On cluster algebras arising from unpunctured surfaces II

We study cluster algebras with principal and arbitrary coefficient systems that are associated to unpunctured surfaces. We give a direct formula for the Laurent polynomial expansion of cluster variables in these cluster algebras in terms of certain paths on a triangulation of the surface. As an immediate consequence, we prove the positivity conjecture of Fomin and Zelevinsky for these cluster algebras. Furthermore, we obtain direct formulas for F-polynomials and g-vectors and show that F-polynomials have constant term equal to 1. As an application, we compute the Euler-Poincar\'e characteristic of quiver Grassmannians in Dynkin type $A$ and affine Dynkin type $\tilde A$.Comment: 36 pages, 9 figure

Models and theories of lambda calculus

A quarter of century after Barendregt's main book, a wealth of interesting problems about models and theories of the untyped lambda-calculus are still open. In this thesis we will be mainly interested in the main semantics of lambda-calculus (i.e., the Scott-continuous, the stable, and the strongly stable semantics) but we will also define and study two new kinds of semantics: the relational and the indecomposable semantics. Models of the untyped lambda-calculus may be defined either as reflexive objects in Cartesian closed categories (categorical models) or as combinatory algebras satisfying the five axioms of Curry and the Meyer-Scott axiom (lambda-models). Concerning categorical models we will see that each of them can be presented as a lambda-model, even when the underlying category does not have enough points, and we will provide sufficient conditions for categorical models living in arbitrary cpo-enriched Cartesian closed categories to have H^* as equational theory. We will build a categorical model living in a non-concrete Cartesian closed category of sets and relations (relational semantics) which satisfies these conditions, and we will prove that the associated lambda-model enjoys some algebraic properties which make it suitable for modelling non-deterministic extensions of lambda-calculus. Concerning combinatory algebras, we will prove that they satisfy a generalization of Stone representation theorem stating that every combinatory algebra is isomorphic to a weak Boolean product of directly indecomposable combinatory algebras. We will investigate the semantics of lambda-calculus whose models are directly indecomposable as combinatory algebras (the indecomposable semantics) and we will show that this semantics is large enough to include all the main semantics and all the term models of semi-sensible lambda-theories, and that it is however largely incomplete. Finally, we will investigate the problem of whether there exists a non-syntactical model of lambda-calculus belonging to the main semantics which has an r.e. (recursively enumerable) order or equational theory. This is a natural generalization of Honsell-Ronchi Della Rocca's longstanding open problem about the existence of a Scott-continuous model of lambda-beta or lambda-beta-eta. Then, we introduce an appropriate notion of effective model of lambda-calculus, which covers in particular all the models individually introduced in the literature, and we prove that no order theory of an effective model can be r.e.; from this it follows that its equational theory cannot be lambda-beta or lambda-beta-eta. Then, we show that no effective model living in the stable or strongly stable semantics has an r.e. equational theory. Concerning Scott-continuous semantics, we prove that no order theory of a graph model can be r.e. and that many effective graph models do not have an r.e. equational theory.Dans cette thèse on s'intéresse surtout aux sémantiques principales du λ-calcul (c'est- a-dire la sémantique continue de Scott, la sémantique stable, et la sémantique fortement stable) mais on introduit et étudie aussi deux nouvelles sémantiques : la sémantique relationnelle et la sémantique indécomposable. Les modèles du λ-calcul pur peuvent être définis soit comme des objets réflexifs dans des catégories Cartésiennes fermées (modèles catégoriques) soit comme des algèbres combinatoires satisfaisant les cinq axiomes de Curry et l'axiome de Meyer-Scott ( λ-modèles). En ce qui concerne les modèles catégoriques, on montre que tout modèle catégorique peut être présenté comme un λ-modèle, même si la ccc (catégorie Cartésienne fermée) sous-jacente n'a pas assez de points, et on donne des conditions su santes pour qu'un modèle catégorique vivant dans une ccc \cpo-enriched" arbitraire ait H pour théorie équationnelle. On construit un modèle catégorique qui vit dans une ccc d'ensembles et relations (sémantique relationnelle) et qui satisfait ces conditions. De plus, on montre que le λ-modèle associe possède des propriétés algébriques qui le rendent apte a modéliser des extensions non-déterministes du -calcul. En ce qui concerne les algèbres combinatoires, on montre qu'elles satisfont une généralisation du Théorème de Représentation de Stone qui dit que toute algèbre combinatoire est isomorphe a un produit Booléen faible d'algèbres combinatoires directement indécomposables. On étudie la sémantique du λ-calcul dont les modèles sont directement indécomposable comme algèbres combinatoires (sémantique indécomposable); on prouve en particulier que cette sémantique est assez générale pour inclure d'une part les trois sémantiques principales et d'autre part les modèles de termes de toutes les λ-théories semi-sensibles. Par contre, on montre aussi qu'elle est largement incomplète. Finalement, on étudie la question de l'existence d'un modèle non-syntaxique du λ-calcul appartenant aux sémantiques principales et ayant une théorie équationnelle ou inéquationnelle r.e. (récursivement énumérable). Cette question est une généralisation naturelle du problème de Honsell et Ronchi Della Rocca (ouvert depuis plus que vingt ans) concernant l'existence d'un modèle continu de λβ ou λβη. On introduit une notion adéquate de modèles effectifs du λ-calcul, qui couvre en particulier tous les modèles qui ont été introduits individuellement en littérature, et on prouve que la théorie inéquationnelle d'un modèle effectif n'est jamais r.e. ; en conséquence sa théorie équationnelle ne peut pas être λβ ou λβη. On montre aussi que la théorie équationnelle d'un modèle effectif vivant dans la sémantique stable ou fortement stable n'est jamais r.e. En ce qui concerne la sémantique continue de Scott, on démontre que la théorie in équationnelle d'un modèle de graphe n'est jamais r.e. et qu'il existe beaucoup de modèles de graphes effectifs qui ont une théorie équationnelle qui n'est pas r.e

Uniformity in association schemes and coherent configurations: cometric Q-antipodal schemes and linked systems

Inspired by some intriguing examples, we study uniform association schemes and uniform coherent configurations, including cometric Q-antipodal association schemes. After a review of imprimitivity, we show that an imprimitive association scheme is uniform if and only if it is dismantlable, and we cast these schemes in the broader context of certain --- uniform --- coherent configurations. We also give a third characterization of uniform schemes in terms of the Krein parameters, and derive information on the primitive idempotents of such a scheme. In the second half of the paper, we apply these results to cometric association schemes. We show that each such scheme is uniform if and only if it is Q-antipodal, and derive results on the parameters of the subschemes and dismantled schemes of cometric Q-antipodal schemes. We revisit the correspondence between uniform indecomposable three-class schemes and linked systems of symmetric designs, and show that these are cometric Q-antipodal. We obtain a characterization of cometric Q-antipodal four-class schemes in terms of only a few parameters, and show that any strongly regular graph with a ("non-exceptional") strongly regular decomposition gives rise to such a scheme. Hemisystems in generalized quadrangles provide interesting examples of such decompositions. We finish with a short discussion of five-class schemes as well as a list of all feasible parameter sets for cometric Q-antipodal four-class schemes with at most six fibres and fibre size at most 2000, and describe the known examples. Most of these examples are related to groups, codes, and geometries.Comment: 42 pages, 1 figure, 1 table. Published version, minor revisions, April 201

Quiver representations and dimension reduction in dynamical systems

Dynamical systems often admit geometric properties that must be taken into account when studying their behavior. We show that many such properties can be encoded by means of quiver representations. These properties include classical symmetry, hidden symmetry, and feedforward structure, as well as subnetwork and quotient relations in network dynamical systems. A quiver equivariant dynamical system consists of a collection of dynamical systems with maps between them that send solutions to solutions. We prove that such quiver structures are preserved under Lyapunov-Schmidt reduction, center manifold reduction, and normal form reduction

Uniformity in Association schemes and Coherent Configurations: Cometric Q-Antipodal Schemes and Linked Systems

2010 Mathematics Subject Classification. Primary 05E30, Secondary 05B25, 05C50, 51E12