arXiv subjects

Teodor Banica

Publications and source records attributed to Teodor Banica.

96 records · Page 6Linked to original sources

arXiv2000

Subfactors associated to compact Kac algebras

We construct inclusions of the form $(B_0\otimes P)^G\subset (B_1\otimes P)^G$, where $G$ is a compact quantum group of Kac type acting on an inclusion of finite dimensional $\c^*$-algebras $B_0\subset B_1$ and on a $II_1$ factor $P$. Under suitable assumptions on the actions of $G$, this is a subfactor, whose Jones to er and standard invariant can be computed by using techniques of A. Wassermann. The subfactors associated to subgroups of compact groups, to projective representations of compact groups, to finite quantum groups, to finitely generated discrete groups, to vertex models and to spin models are of this form.

math.OA↗

arXiv1999

Compact Kac algebras and commuting squares

We consider commuting squares of finite dimensional von Neumann algebras having the algebra of complex numbers in the lower left corner. Examples include the vertex models, the spin models (in the sense of subfactor theory) and the commuting squares associated to finite dimensional Kac algebras. To any such commuting square we associate a compact Kac algebra and we compute the corresponding subfactor and its standard invariant in terms of it.

math.OA↗

arXiv1999

Fusion rules for representations of compact quantum groups

We give a survey of some recent results on the fusion semirings of compact quantum groups (computations of and applications to discrete quantum groups) by using the following simplifying terminology: we say that a compact quantum group G is an R^+-deformation of a compact quantum group H if their fusion semirings are isomorphic. The paper contains also some easy related results (with proofs), two conjectures and many remarks and comments, some of them concerning classification by invariants related to R^+.

math.QA↗

arXiv1999

Symmetries of a generic coaction

If B is C*-algebra of finite dimension n>3 then the finite dimensional irreducible representations of the compact quantum automorphism group of B, say G, have the same fusion rules as the ones of SO(3). As consequences, we get (1) a structure result for G in the case where B is a matrix algebra (2) if n>4 then the dual of G is not amenable (3) the fixed point subfactor P^G\subset (B\otimes P)^G has index n and principal graph A_\infty.

math.QA↗

arXiv1998

Representations of compact quantum groups and subfactors

We associate Popa systems (= standard invariants of subfactors) to the finite dimensional representations of compact quantum groups. We characterise the systems arising in this way: these are the ones which can be ``represented'' on finite dimensional Hilbert spaces. This is proved by an universal construction. We explicitely compute (in terms of some free products) the operation of going from representations of compact quantum groups to Popa systems and then back via the universal construction. We prove a Kesten type result for the co-amenability of compact quantum groups, which allows us to compare it with the amenability of subfactors.

math.QA↗

arXiv1998

Hopf algebras and subfactors associated to vertex models

If H is a Hopf algebra whose square of the antipode is the identity, $v\inł(V)\otimes H$ is a corepresentation, and $π:H\toł(W)$ is a representation, then $u=(id\otimesπ)v$ satisfies the equation $(t\otimes id)u^{-1}=((t\otimes id)u)^{-1}$ of the vertex models for subfactors. A universal construction shows that any solution $u$ of this equatio n arises in this way. A more elaborate construction shows that there exists a ``minimal'' triple $(H,v,π)$ satisfying $(id\otimesπ)v=u$. This paper is devoted to the study of this latter construction of Hopf algebras. If $u$ is unitary we construct a $\c^*$-norm on $H$ and we find a new description of the standard invariant of the subfactor associated to $u$. We discuss also the ``twisted'' (i.e. $S^2\neq id$) case.

math.QA↗