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We show that the quantum coordinate ring of the unipotent subgroup N(w) of a symmetric Kac-Moody group G associated with a Weyl group element w has the structure of a quantum cluster algebra. This quantum cluster structure arises naturally from a subcategory C_w of the module category of the corresponding preprojective algebra. An important ingredient of the proof is a system of quantum determinantal identities which can be viewed as a q-analogue of a T-system. In case G is a simple algebraic group of type A, D, E, we deduce from these results that the quantum coordinate ring of an open cell of a partial flag variety attached to G also has a cluster structure., Comment: v2: minor corrections. v3: references updated, final version to appear in Selecta Mathematica

Starting from known $q$-analogues of ordinary SU(n) tensor products multiplicities, we introduce $q$-analogues of the fusion coefficients of the WZW conformal field theories associated with SU(n). We conjecture combinatorial interpretations of these polynomials, which can be proved in special cases. This allows us to derive in a simple way various kinds of branching functions, the simplest ones being the characters of the minimal unitary series of the Virasoro algebra. We also obtain $q$-analogues of the dimensions of spaces of nonabelian theta functions., Comment: 12 pages, Latex, epsf and iop macros. Extended abstract of a talk presented by J.-Y. T. at the conference "Symmetry and Structural Properties of Condensed Matter" (Zajackowo, August 27-September 2, 1998)

We give a combinatorial algorithm for computing Zelevinsky's involution of the set of isomorphism classes of irreducible representations of the affine Hecke algebra $\H_m(t)$ when $t$ is a primitive $n$th root of 1. We show that the same map can also be interpreted in terms of aperiodic nilpotent orbits of $\Zb/n\Zb$-graded vector spaces., Comment: 17 pages, Latex, epsf macros

We study the restrictions of simple modules of Ariki-Koike algebras $\H_m(\v)$ with set of parameters $\v= (\zeta;\zeta^{v_0},... ,\zeta^{v_{l-1}})$, where $\zeta$ is an $n$th root of unity, to their subalgebras $\H_{m-j}(\v)$. Using a theorem of Ariki and the crystal basis theory of Kashiwara, we relate this problem to the calculation of tensor product multiplicities of highest weight irreducible representations of the affine Lie algebra $A_{n-1}^{(1)}$. These multiplicities have a combinatorial description in terms of higher level paths or highest-lift multipartitions. This enables us to solve the Jantzen-Seitz problem for Ariki-Koike algebras, that is, to determine which irreducible representations of $\H_m(\v)$ restrict to irreducible representations of $\H_{m-1}(\v)$. From a combinatorial point of view, this problem is identical to that of computing the tensor product of an $A_{n-1}^{(1)}$-module of level $l$ and one of level 1. We also consider natural generalisations of the Jantzen-Seitz problem corresponding to the product of a level $l$ module by a level $l'>1$ module, and from the commutativity of tensor products, we deduce a remarkable symmetry between the generalised Jantzen-Seitz conditions and the sets of parameters of the Ariki-Koike algebras., Comment: 35 pages, Latex, epsf macros

We use the Fock space representation of the quantum affine algebra of type $A^{(2)}_{2n}$ to obtain a description of the global crystal basis of its basic level 1 module. We formulate a conjecture relating this basis to decomposition matrices of spin symmetric groups in characteristic $2n+1$., Comment: 18 pages, Latex, epsf and iop macros (ioplppt.sty). Revised version, to appear in J. Phys. A. Some typos corrected, and some comments added in the introduction

We show that the quantum straightening algorithm for Young tableaux and Young bitableaux reduces in the crystal limit $q \mapsto 0$ to the Robinson-Schensted algorithm., Comment: LaTeX file with eepic pictures, 23 pages

We give an expression of the $q$-analogues of the multiplicities of weights in irreducible $\sl_{n+1}$-modules in terms of the geometry of the crystal graph attached to the corresponding $U_q(\sl_{n+1})$-modules. As an application, we describe multivariate polynomial analogues of the multiplicities of the zero weight, refining Kostant's generalized exponents., Comment: LaTeX file with epic, eepic pictures, 17 pages, November 1994, to appear in Lett. Math. Phys

We present several identities involving quasi-minors of noncommutative generic matrices. These identities are specialized to quantum matrices, yielding q-analogues of various classical determinantal formulas., Comment: 24 pages tex dialect: LaTex

This paper presents a noncommutative theory of symmetric functions, based on the notion of quasi-determinant. We begin with a formal theory, corresponding to the case of symmetric functions in an infinite number of independent variables. This allows us to endow the resulting algebra with a Hopf structure, which leads to a new method for computing in descent algebras. It also gives unified reinterpretation of a number of classical constructions. Next, we study the noncommutative analogs of symmetric polynomials. One arrives at different constructions, according to the particular kind of application under consideration. For example, when a polynomial with noncommutative coefficients in one central variable is decomposed as a product of linear factors, the roots of these factors differ from those of the expanded polynomial. Thus, according to whether one is interested in the construction of a polynomial with given roots or in the expansion of a product of linear factors, one has to consider two distinct specializations of the formal symmetric functions. A third type appears when one looks for a noncommutative generalization of applications related to the notion of characteristic polynomial of a matrix. This construction can be applied, for instance, to the noncommutative matrices formed by the generators of the universal enveloping algebra $U(gl_n)$ or of, Comment: 111 pages