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In the thesis we acquaint ourselves with the terms from ergodic theory and re- presentation theory of topological groups. We pay attention particularly to terms unitary representation, realizability by an action, dual group, unitary equivalence and Kazhdan's property (T). We achieve a result regarding unitary representati- ons realizable by an action on finite abelian groups according to article [5] and show that it is possible to generalize it to all finite groups at the end of the thesis according to article [6]. A large part of the text subsequently deals with proper- ties of unitary representations and their relations. We connect the terms compact topological group and Kazhdan's property (T).

We investigate unitarisability of groups by looking at induced actions on the cone of positive operators.

In dieser Dissertation untersuchen wir das Verhalten von Schrödinger-Operatoren unter Riemannschen Überlagerungen. Wir betrachten folgende Situation: Sei eine Riemannsche Überlagerung und ein Schrödinger-Operator S mit glattem, von unten beschränktem Potential auf der Basismannigfaltigkeit gegeben. Sei S‘ der Lift von S auf die Überlagerungsmannigfaltigkeit. Man sieht leicht, dass das Minimum des Spektrums von S nicht größer als das Minimum des Spektrums von S‘ ist. R. Brooks hat als erster untersucht, wann die Gleichheit gilt. Er bewies insbesondere, dass eine normale Riemannsche Überlagerung einer geschlossenen Mannigfaltigkeit genau dann amenabel ist, wenn sie das Minimum des Spektrums des Laplace-Operators unverändert lässt. Zusammen mit W. Ballmann und H. Matthiesen bewiesen wir, dass amenable Riemannsche Überlagerungen immer das Minimum des Spektrums von Schrödinger-Operatoren erhalten; dies verallgemeinert Resultate von R. Brooks sowie von P. Bérard und Ph. Castillon. In dieser Dissertation beweisen wir, dass im Fall vollständiger Mannigfaltigkeiten das Spektrum von S im Spektrum von S‘ enthalten ist. Tatsächlich beweisen wir diese Beziehung sogar für eine deutlich größere Klasse von Differentialoperatoren. Obwohl Amenabilität eine natürliche Bedingung für die Gleichheit der Minima der Spektren ist, ist es unklar, inwieweit diese Bedingung optimal ist. In dieser Dissertation beweisen wir: Wenn eine Riemannsche Überlagerung das Minimum des Spektrums eines Schrödinger-Operators erhält, und wenn dieses zum diskreten Spektrum des Operators auf der Basismannigfaltigkeit gehört, dann ist die Überlagerung amenabel. Man beachte, dass wir keinerlei geometrische oder topologische Bedingungen an die Mannigfaltigkeiten stellen. Dies verallgemeinert sowohl frühere Resultate von R. Brooks, T. Roblin und S. Tapie als auch ein kürzliches Resultat aus einer gemeinsamen Arbeit mit W. Ballmann und H. Matthiesen. / In this thesis, we investigate the behavior of the spectrum of Schrödinger operators under Riemannian coverings. To set the stage, consider a Riemannian covering and a Schrödinger operator S on the base manifold, with smooth potential bounded from below potential. Let S’ be the lift of S on the covering space. It is easy to see that the bottom (that is, the minimum) of the spectrum of S is no greater than the bottom of the spectrum of S’. R. Brooks was the first one to examine when the equality holds. In particular, he proved that a normal Riemannian covering of a closed manifold is amenable if and only if it preserves the bottom of the spectrum of the Laplacian. Generalizing former results of R. Brooks, and P. Berard and Ph. Castillon, in a joint work with W. Ballmann and H. Matthiesen, we proved that amenable Riemannian coverings preserve the bottom of the spectrum of Schrödinger operators. In this thesis, we prove that if, in addition, the manifolds are complete, then the spectrum of S is contained in the spectrum of S’. As a matter of fact, we establish this result for a quite wide class of differential operators. Although amenability is a natural assumption for the preservation of the bottom of the spectrum, it is not clear to what extent it is optimal. In this thesis, we prove that if a Riemannian covering preserves the bottom of the spectrum of a Schrödinger operator, which belongs to the discrete spectrum of the operator on the base manifold, then the covering is amenable. It is worth to point out that we do not impose any geometric or topological assumptions on the manifolds. This generalizes former results by R. Brooks, T. Roblin and S. Tapie, and a recent result of a joint work with W. Ballmann and H. Matthiesen.

This thesis is devoted to the study of operators on discrete structures. The operators are supposed to be self-adjoint and obey a certain translation invariance property. The discrete structures are given as Cayley graphs via finitely generated groups. Here, sofic groups and amenable groups are in the center of our considerations. Note that every finitely generated amenable group is sofic. We investigate the spectrum of a discrete self-adjoint operator by studying a sequence of finite dimensional analogues of these operators. In the setting of amenable groups we obtain these approximating operators by restricting the operator in question to finite subsets Qn , n ∈ N. These finite dimensional operators are self-adjoint and therefore admit a well-defined normalized eigenvalue counting function. The limit of the normalized eigenvalue counting functions when |Qn | → ∞ (if it exists) is called the integrated density of states (IDS). It is a distribution function of a probability measure encoding the distribution of the spectrum of the operator in question on the real axis. In this thesis, we prove the existence of the IDS in various geometric settings and for different types of operators. The models we consider include deterministic as well as random situations. Depending on the specific setting, we prove existence of the IDS as a weak limit of distribution functions or even as a uniform limit. Moreover, in certain situations we are able to express the IDS via a semi-explicit formula using the trace of the spectral projection of the original operator. This is sometimes referred to as the validity of the Pastur-Shubin trace formula. In the most general geometric setting we study, the operators are defined on Cayley graphs of sofic groups. Here we prove weak convergence of the eigenvalue counting functions and verify the validity of the Pastur-Shubin trace formula for random and non-random operators . These results apply to operators which not necessarily bounded or of finite hopping range. The methods are based on resolvent techniques. This theory is established without having an ergodic theorem for sofic groups at hand. Note that ergodic theory is the usual tool used in the proof of convergence results of this type. Specifying to operators on amenable groups we are able to prove stronger results. In the discrete case, we show that the IDS exists uniformly for a certain class of finite hopping range operators. This is obtained by using a Banach space-valued ergodic theorem. We show that this applies to eigenvalue counting functions, which implies their convergence with respect to the Banach space norm, in this case the supremum norm. Thus, the heart of this theory is the verification of the Banach space-valued ergodic theorem. Proceeding in two steps we first prove this result for so-called ST-amenable groups. Then, using results from the theory of ε-quasi tilings, we prove a version of the Banach space-valued ergodic theorem which is valid for all amenable groups. Focusing on random operators on amenable groups, we prove uniform existence of the IDS without the assumption that the operator needs to be of finite hopping range or bounded. Moreover, we verify the Pastur-Shubin trace formula. Here we present different techniques. First we show uniform convergence of the normalized eigenvalue counting functions adapting the technique of the Banach space-valued ergodic theorem from the deterministic setting. In a second approach we use weak convergence of the eigenvalue counting functions and additionally obtain control over the convergence at the jumps of the IDS. These ingredients are applied to verify uniform existence of the IDS. In both situations we employ results from the theory of large deviations, in order to deal with long-range interactions.

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Une partie symétrique X d'un groupe G est un sous-groupe K-approximatif s'il existe une partie finie E ⊂ G de taille K telle que X2 ⊂ E.X. L'étude combinatoire des groupes approximatifs a grandement bénéficié des apports de la Théorie des Modèles : en 2009, Hrushovski montre qu'une ultralimite de groupes approximatifs finis possède une composante connexe modèle-théorique, donc un quotient localement compact X/H. En appliquant les résultats de Gleason et Yamabe sur le cinquième problème de Hilbert, cela permet de trouver un morphisme vers un groupe de Lie, et d'en déduire des résultats de nilpotence. Cela a permis à Breuillard, Green et Tao de classifier tous les groupes approximatifs finis, en retrouvant un quotient X/H de manière combinatoire. Dans cette thèse, on s'intéresse à la construction d'un sous-groupe H type-définissable et d'indice borné, qui garantit l'existence d'un quotient localement compact. On montre que l'approche combinatoire de Breuillard, Green et Tao peut être vue de cette manière, et on la généralise à tous les groupes approximatifs définissablement moyennables. On montre aussi que si H est type-définissable dans un langage L∗, alors on peut construire un sous-groupe H qui est type-définissable sur un langage réduit L, et toujours d'indice borné. L'existence de H ne dépend donc pas du choix du langage / A symmetric subset X in a group G is a K-approximate subgroup if there exists a finite set E ⊂ G of cardinality K such that X2 ⊂ E.X. The study of approximate subgroups in multiplicative combinatorics experienced a significate advance through the use of model theory. In 2009, Hrushovski showed that an ultralimit of finite approximate subgroups has a model-theoretic connected component, thus a locally compact quotient X/H. Using the results of Gleason and Yamabe about Hilbert’s fifth problem, this allows the construction of a morphism to a Lie group, and deduce some results about nilpotency. This lead to the theorem of Breuillard, Green and Tao classifying all finite approximate subgroups, using a combinatorial construction of the quotient X/H. In this thesis, we are intersested in the conditions needed to construct a type definable subgroup H of bounded index in X. This implies the existence of a locally compact quotient.We show that the combinatorial construction of Breuillard, Green and Tao can be seen in a definable way, and give a generalisation to all definably amenable approximate subgroups. Also, we show that if H is type-definable in a language L∗, then it is possible to construct a subgroup H which is type-definable in a reduct L, still with bounded index. Thus the existence of a subgroup H does not depend on the choice of a base language.