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## Macdonald polynomials and extended Gelfand-Tsetlin graph

Using Okounkov's *q*-integral representation of Macdonald polynomials we construct an infinite sequence Ω1,Ω2,Ω3,… of countable sets linked by transition probabilities from Ω*N* to Ω*N*−1 for each *N*=2,3,…. The elements of the sets Ω*N* are the vertices of the extended Gelfand-Tsetlin graph, and the transition probabilities depend on the two Macdonald parameters, *q* and *t*. These data determine a family of Markov chains, and the main result is the description of their entrance boundaries. This work has its origin in asymptotic representation theory. In the subsequent paper, the main result is applied to large-*N* limit transition in (*q*,*t*)-deformed *N*-particle beta-ensembles.

This is a survey of the author's and his collaboratots' recent works on the quasiflags' moduli spaces introduced by Gerard Laumon some 25 years ago. These spaces are used in the study of geometric Eisenstein series, quantum cohomology and K-theory of the flag varieties, Weyl modules, Nekrasov partition function of N=2 supersymmetric gauge quantum field theory.

Let G be an almost simple simply connected complex Lie group, and let G/U be its base affine space. In this paper we formulate a conjecture which provides a new geometric interpretation of the Macdonald polynomials associated to G via perverse coherent sheaves on the scheme of formal arcs in the affinizationof G/U. We prove our conjecture for G=SL(N) using the so called Laumon resolution of the space of quasimaps. In the course of the proof we also give a K-theoretic version of the main result of Negut.

We introduce and study a family of (*q*, *t*)-deformed discrete *N*-particle beta ensembles, where *q* and *t* are the parameters of Macdonald polynomials. The main result is the existence of a large-*N* limit transition leading to random point processes with infinitely many particles.

Let G be an almost simple simply connected complex Lie group, and let G/U− be its base affine space. In this paper we formulate a conjecture, which provides a new geometric interpretation of the Macdonald polynomials associated to G via perverse coherent sheaves on the scheme of formal arcs in the affinization of G/U−. We prove our conjecture for G = SL(N) using the so called Laumon resolution of the space of quasi-maps (using this resolution one can reformulate the statement so that only “usual” (not perverse) coherent sheaves are used). In the course of the proof we also give a K-theoretic version of the main result of Negut (2009).

A bstract We continue our study of the AGT correspondence between instanton counting on ${{{{{\ mathbb {C}}^ 2}}}\ left/{{{{\ mathbb {Z}} _p}}}\ right.} $ and Conformal field theories with the symmetry algebra $\ mathcal {A}\ left ({r, p}\ right) $. In the cases r= 1, p= 2 and r= 2, p= 2 this algebra specialized to: $\ mathcal {A}\ left ({1, 2}\ right)=\ mathcal {H}\ oplus\ widehat {\ mathfrak {sl}}{(2) _1} $ and $\ mathcal {A}\ left ({2, 2}\ right)=\ mathcal {H}\ oplus\ widehat {\ mathfrak {sl}}{(2) _2}\ oplus\ mathrm {NSR} $.

A model for organizing cargo transportation between two node stations connected by a railway line which contains a certain number of intermediate stations is considered. The movement of cargo is in one direction. Such a situation may occur, for example, if one of the node stations is located in a region which produce raw material for manufacturing industry located in another region, and there is another node station. The organization of freight traﬃc is performed by means of a number of technologies. These technologies determine the rules for taking on cargo at the initial node station, the rules of interaction between neighboring stations, as well as the rule of distribution of cargo to the ﬁnal node stations. The process of cargo transportation is followed by the set rule of control. For such a model, one must determine possible modes of cargo transportation and describe their properties. This model is described by a ﬁnite-dimensional system of diﬀerential equations with nonlocal linear restrictions. The class of the solution satisfying nonlocal linear restrictions is extremely narrow. It results in the need for the “correct” extension of solutions of a system of diﬀerential equations to a class of quasi-solutions having the distinctive feature of gaps in a countable number of points. It was possible numerically using the Runge–Kutta method of the fourth order to build these quasi-solutions and determine their rate of growth. Let us note that in the technical plan the main complexity consisted in obtaining quasi-solutions satisfying the nonlocal linear restrictions. Furthermore, we investigated the dependence of quasi-solutions and, in particular, sizes of gaps (jumps) of solutions on a number of parameters of the model characterizing a rule of control, technologies for transportation of cargo and intensity of giving of cargo on a node station.

Let k be a field of characteristic zero, let G be a connected reductive algebraic group over k and let g be its Lie algebra. Let k(G), respectively, k(g), be the field of k- rational functions on G, respectively, g. The conjugation action of G on itself induces the adjoint action of G on g. We investigate the question whether or not the field extensions k(G)/k(G)^G and k(g)/k(g)^G are purely transcendental. We show that the answer is the same for k(G)/k(G)^G and k(g)/k(g)^G, and reduce the problem to the case where G is simple. For simple groups we show that the answer is positive if G is split of type A_n or C_n, and negative for groups of other types, except possibly G_2. A key ingredient in the proof of the negative result is a recent formula for the unramified Brauer group of a homogeneous space with connected stabilizers. As a byproduct of our investigation we give an affirmative answer to a question of Grothendieck about the existence of a rational section of the categorical quotient morphism for the conjugating action of G on itself.

Let G be a connected semisimple algebraic group over an algebraically closed field k. In 1965 Steinberg proved that if G is simply connected, then in G there exists a closed irreducible cross-section of the set of closures of regular conjugacy classes. We prove that in arbitrary G such a cross-section exists if and only if the universal covering isogeny Ĝ → G is bijective; this answers Grothendieck's question cited in the epigraph. In particular, for char k = 0, the converse to Steinberg's theorem holds. The existence of a cross-section in G implies, at least for char k = 0, that the algebra k[G]G of class functions on G is generated by rk G elements. We describe, for arbitrary G, a minimal generating set of k[G]G and that of the representation ring of G and answer two Grothendieck's questions on constructing generating sets of k[G]G. We prove the existence of a rational (i.e., local) section of the quotient morphism for arbitrary G and the existence of a rational cross-section in G (for char k = 0, this has been proved earlier); this answers the other question cited in the epigraph. We also prove that the existence of a rational section is equivalent to the existence of a rational W-equivariant map T- - - >G/T where T is a maximal torus of G and W the Weyl group.