A theorem of Heine-Stieltjes, the Wronski map, and Bethe vectors in the sl_p Gaudin model

Abstract

Heine and Stieltjes in their studies of linear second-order differential equations with polynomial coefficients having a polynomial solution of a preassigned degree, discovered that the roots of such a solution are the coordinates of a critical point of a certain remarkable symmetric function, [He], [St]. Their result can be reformulated in terms of the Schubert calculus as follows: the critical points label the elements of the intersection of certain Schubert varieties in the Grassmannian of two-dimensional subspaces of the space of complex polynomials, [S1]. In a hundred years after the works of Heine and Stieltjes, it was established that the same critical points determine the Bethe vectors in the sl_2 Gaudin model, [G]. Recently it was proved that the Bethe vectors of the sl_2 Gaudin model form a basis of the subspace of singular vectors of a given weight in the tensor product of irreducible sl_2-representations, [SV]. In the present work we generalize the result of Heine and Stieltjes to linear differential equations of order p>2. The function, which determines elements in the intersection of corresponding Schubert varieties in the Grassmannian of p-dimensional subspaces, turns out to be the very function which appears in the sl_p Gaudin model. In the case when the space of states of the Gaudin model is the tensor product of symmetric powers of the standard sl_p-representation, we prove that the Bethe vectors form a basis of the subspace of singular vectors of a given weight.