Geometry, Combinatorics, and Integrable SystemsAutumn 2017Time: Thursdays 34pmLocation: MA 317 

August 31
Thurs, 3pm  Max Glick
(OSU) 
The limit point of the pentagram map 
September 21
Thurs, 3pm  Yuancheng Xie
(OSU) 
Generalized hypergeometric functions on the Grassmannian and integrable systems of hydrodynamic type 
September 28
Thurs, 3pm  Ed Richmond
(Oklahoma State) 
Pattern avoidance and fiber bundle structures on Schubert varieties 
October 5
Thurs, 3pm  Tair Akhmejanov
(Cornell) 
Affine Growth Diagrams 
October 12
Thurs, 3pm  (fall break)


October 19
Thurs, 3pm MW 154  Cristian Lenart
(Albany) 
Combinatorics of Lusztig's tanalogue of weight multiplicity 
(Glick): The pentagram map is a discrete dynamical system defined on the space of polygons in the plane. In the first paper on the subject, R. Schwartz proved that the pentagram map produces from each convex polygon a sequence of successively smaller polygons that converges exponentially to a point. We investigate the limit point itself, giving an explicit description of its Cartesian coordinates as roots of certain degree three polynomials.
(Xie): In this talk I will describe a connection between generalized hypergeometric functions on the Grassmannian and integrable systems of hydrodynamic type. This talk consists of the following four parts: (1) Symmetries of hydrodynamictype systems in Riemann invariant form; (2) Generalized hypergeometric functions defined on the Grassmannian and their confluences; (3) Integrable hydrodynamictype systems and confluences of Lauricellatype hypergeometric functions; (4) Some preliminary results on generalizations to the Grassman Gr(r,n) with r > 2. A special case of the compatibility conditions of hydrodynamictype systems in Riemann invariant form leads to EulerPoissonDarboux(EPD) system. This system admits Lauricella type hypergeometric functions as solutions, which are special cases of AomotoGel'fand hypergeometric functions defined on the Grassman Gr(r, n) with r=2. In this way each Lauricella type hypergeometric functions gives rise to a hierarchy of integrable systems of hydrodynamic type in Riemann invariant form. The confluences of the classical Gauss hypergeometric functions can be generalized to AomotoGel'fand hypergeometric functions. The confluences of a generalized Lauricella type hypergeometric function also produce hierarchies of integrable systems of hydrodynamic type which are not necessarily in Riemann invariant form anymore. In this talk I will describe these constructions and connections and also some preliminary results on generalizations to the Grassman Gr(r,n) with r>2.
(Richmond): In this talk, I will discuss joint work with Tim Alland where we give a pattern avoidance criteria for determining when the projection map from the flag variety to a Grassmannian induces a fiber bundle structure on a Schubert variety. To do this, we introduce the notion of split pattern avoidance and show that a Schubert variety has such a fiber bundle structure if and only if the corresponding permutation avoids the split patterns 312 and 231. Continuing, we also characterize when a Schubert variety is an iterated fiber bundle of Grassmannian Schubert varieties in terms of usual pattern avoidance.
(Akhmejanov): We introduce a new type of growth diagram, arising from the geometry of the affine Grassmannian for GL_{m}. These affine growth diagrams are in bijection with the c_{lambda} many components of the polygon space Poly(lambda) for lambda a sequence of minuscule weights and c_{lambda} the LittlewoodRichardson coefficient. Unlike Fomin growth diagrams, they are infinite periodic on a staircase shape, and each vertex is labeled by a dominant weight of GL_{m}. Letting $m$ go to infinity, a dominant weight can be viewed as a pair of partitions, and we recover the RSK correspondence and Fomin growth diagrams within affine growth diagrams. The main combinatorial tool used in the proofs is the nhive of KnutsonTaoWoodward. The local growth rule previously appeared in van Leeuwen's work on Littelmann paths, so our results give a geometric interpretation of this combinatorial rule.
(Lenart): Lusztig defined the KostkaFoulkes polynomial K_{λ,μ}(t) as a tanalogue of the multiplicity of a weight μ in the irreducible representation of highest weight λ of a semisimple Lie algebra. This polynomial has remarkable properties, such as being an affine KazhdanLusztig polynomial. Finding combinatorial formulas for K_{λ,μ}(t) beyond type A_{n} has been a longstanding problem. In joint work with Cédric Lecouvey, we give the first such formula, for K_{λ,0}(t) in type C_{n}, using combinatorics of Kashiwara's crystal graphs; the special case μ=0 is, in fact, the most complex one. Related aspects and applications will be discussed. I will also mention the socalled atomic decomposition of KostkaFoulkes polynomials, as well as its relevance to the geometric construction of representations given by the Satake correspondence. The talk will be largely selfcontained.