A marvellous embedding of the Lagrangian Grassmannian

Abstract

We give a embedding of the Lagrangian Grassmannian LG(n) inside an ordinary Grassmannian that is well-behaved with respect to the Wronski map. As a consequence, we obtain an analogue of the Mukhin-Tarasov-Varchenko theorem for LG(n). The restriction of the Wronski map to LG(n) has degree equal to the number of shifted or unshifted tableaux of staircase shape. For special fibres one can define bijections, which, in turn, gives a bijection between these two classes of tableaux. The properties of these bijections lead a geometric proof of a branching rule for the cohomological map H*(Gr(n,2n)) x H*(LG(n)) -> H*(LG(n)), induced by the diagonal inclusion LG(n) -> LG(n) x Gr(n,2n). We also discuss applications to the orbit structure of jeu de taquin promotion on staircase tableaux.