Irreducible projective representations of finite groups

The purpose of this paper is to prove a result on the number of irreducible projective representations of a finite group with respect to a given factor set and a group of linear characters acting on them. It includes a determination of the number of classes of projectively equivalent representations as well as a result of M. Osima on the classes of linearly equivalent representations. Osima proved his result by defining and calculating on irreducible projective Brauer characters, whereas the method used here is based on Brauer's well-known result for the linear modular representations and an induction argument on suitable coverings.     

On projective representations for compact quantum groups

We study actions of compact quantum groups on type I-factors, which may be interpreted as projective representations of compact quantum groups. We generalize to this setting some of Woronowicz?s results concerning Peter-Weyl theory for compact quantum groups. The main new phenomenon is that for general compact quantum groups (more precisely, those which are not of Kac type), not all irreducible projective representations have to be finite-dimensional. As applications, we consider the theory of projective representations for the compact quantum groups associated with group von Neumann algebras of discrete groups, and consider a certain non-trivial projective representation for quantum SU(2).     

Injectivity, Projectivity and Supercuspidal Representations

Let G be a reductive p-adic group. Consider the category ofsmooth (complex) representations of G in which a (fixed) closedcocompact subgroup of the centre acts by a (fixed) character.It is well known that the supercuspidal representations in thiscategory are both injective and projective. It is shown that,conversely, an admissible injective or projective object isnecessarily supercuspidal.     

Projective representations II. generalized chain geometries

In this paper, projective representations of generalized chain geometries are investigated, using the concepts and results of [5]. In particular, we study under which conditions such a projective representation maps the chains of a generalized chain geometry Σ (F, R) to reguli; this mainly depends on how the field F is embedded in the ring R. Moreover, we determine all bijective morphisms of a certain class of generalized chain geometries with the help of projective representations. Dedicated to Walter Benz on the occasion of his 70th birthday. The first author was supported by a Lise Meitner Research Fellowship of the Austrian Science Fund (FWF), projects M529-MAT, M574-MAT..     

Noncomputability of classes of pappian and desarguesian projective planes

We study computable representations of projective planes and prove that the class of all pappian projective planes and the class of all desarguesian projective planes have no computable numberings (up to computable isomorphism).     

Central Pairs, Galois Theory and Automorphic Forms

A central pair over a field k of characteristic 0 consists of a finite Abelian group which is equipped with a central 2-cocycle with values in the multiplicative group k ^* of k. In this paper we use specific central pairs to construct a class of projective representations of the absolute Galois group G _k of k and if k is a number field we investigate the liftings of these projective representations to linear representations of G _k . In particular we relate these linear representations to automorphic representations. It turns out that some of these automorphic representations correspond to certain indefinite modular forms already constructed by E. Hecke.     

On the representations of projective geometries in algebraic combinatorial geometries

In this paper, we show that the full algebraic combinatorial geometry is not a projective geometry, it is only semimodular, but the p-polynomial points give a projective subgeometry. Also, we show that the subgeometry can be coordinatized by a skew field, which is quotient ring of an Ore domain. As a corollary, we prove the existence of algebraic representations over fields of prime characteristic of the non-Pappus matroid and its dual matroid. Regarding the existence of algebraic representations of the non-Pappus matroid, this result was earlier proved by Lindström [7] for finite fields.     

Partial projective representations and partial actions

The concept of a partial projective representation of a group is introduced and studied. The interaction with partial actions is explored. It is shown that the factor sets of partial projective representations over a field K are exactly the K-valued twistings of crossed products by partial actions.     

On quasifree representations of infinite dimensional symplectic group

We consider an infinite dimensional generalization of metaplectic representations (Weil representations) for the (double covering of) symplectic group. Given quasifree states of an infinite dimensional CCR algebra, projective unitary representations of the infinite dimensional symplectic group are constructed via unitary implementors of Bogoliubov automorphisms. Complete classification of these representations up to quasi-equivalence is obtained.     

Cyclic coverings of the <Emphasis Type="Italic">p</Emphasis>-adic projective line by Mumford curves

Exact bounds for the positions of the branch points for cyclic coverings of the p-adic projective line by Mumford curves are calculated in two ways. Firstly, by using Fumiharu Kato’s *-trees, and secondly by giving explicit matrix representations of the Schottky groups corresponding to the Mumford curves above the projective line through combinatorial group theory.     

The Deformation Spaces of Projective Structures on 3-Dimensional Coxeter Orbifolds

A real projective structure on a 3-orbifold is given by locally modeling the orbifold by real projective geometry. We present some methodology to study Coxeter groups which are fundamental groups of 3-orbifolds with representations in and deformation spaces. There are related examples by Benoist. These examples give us nontrivial deformation spaces of projective structures.     

On the sign of regular algebraic polarizable automorphic representations

We remove a parity condition from the construction of automorphic Galois representations carried out in the Paris Book Project. We subsequently generalize this construction to the case of ‘mixed-parity’ (but still regular essentially self-dual) automorphic representations over totally real fields, finding associated geometric projective representations. Finally, we optimize some of our previous results on finding geometric lifts, through central torus quotients, of geometric Galois representations, and apply them to the previous mixed-parity setting.     

Virtually abelian Kähler and projective groups

We characterise the virtually abelian groups which are fundamental groups of compact Kähler manifolds and of smooth projective varieties. We show that a virtually abelian group is Kähler if and only if it is projective. In particular, this allows to describe the Kähler condition for such groups in terms of integral symplectic representations.     

Projective representations i. projective lines over rings

We discuss representations of the projective line over a ringR with 1 in a projective space over some (not necessarily commutative) fieldK. Such a representation is based upon a (K, R)-bimoduleU. The points of the projective line overR are represented by certain subspaces of the projective space ℙ(K, U ×U) that are isomorphic to one of their complements. In particular, distant points go over to complementary subspaces, but in certain cases, also non-distant points may have complementary images.     

103-configurations and projective realizability of multiplied configurations

Some remarks on 10₃-configurations which contain the complete graph K ₄ are given, on their representations, and on projective realizability. Results are applied to show a class of configurations that cannot be realized in any Pappian projective space.      

Phase Retrievable Projective Representation Frames for Finite Abelian Groups

Journal of Fourier Analysis and Applications - We consider the problem of characterizing projective representations that admit frame vectors with the maximal span property, a property that allows...     

Complexity of the Isomorphism Problem for Computable Free Projective Planes of Finite Rank

Studying computable representations of projective planes, we prove that the isomorphism problem in the class of free projective planes of finite rank is an m-complete Δ⁰₃-set within the class.