Isotropic Grassmannians, Plücker and Cartan maps

Abstract

This work is motivated by the relation between the KP and BKP integrable hierarchies, whose τ-functions may be viewed as flows of sections of dual determinantal and Pfaffian line bundles over infinite dimensional Grassmannians. In finite dimensions, we show how to relate the Cartan map, which, for a vector space V of dimension N, embeds the Grassmannian GrV0(V+V*) of maximal isotropic subspaces of V + V*, with respect to the natural scalar product, into the projectivization of the exterior space Λ(V), and the Plücker map, which embeds the Grassmannian GrV(V + V*) of all N-planes in V + V* into the projectivization of ΛN(V + V*). The Plücker coordinates on GrV0(V+V*) are expressed bilinearly in terms of the Cartan coordinates, which are holomorphic sections of the dual Pfaffian line bundle Pf*→GrV0(V+V*,Q). In terms of affine coordinates on the big cell, this is equivalent to an identity of Cauchy–Binet type, expressing the determinants of square submatrices of a skew symmetric N × N matrix as bilinear sums over the Pfaffians of their principal minors.