B-orbits of square zero in nilradical of the symplectic algebra

Abstract

Let $SP_n(\mathbb{C})$ be the symplectic group and $\mathfrak{sp}_n(\mathbb{C})$ its Lie algebra. Let $B$ be a Borel subgroup of $SP_n(\mathbb{C} )$, $\mathfrak{b}={\rm Lie}(B)$ and $\mathfrak n$ its nilradical. Let $\mathcal X$ be a subvariety of elements of square 0 in $\mathfrak n.$ $B$ acts adjointly on $\mathcal X$. In this paper we describe topology of orbits $\mathcal X/B$ in terms of symmetric link patterns. Further we apply this description to the computations of the closures of orbital varieties of nilpotency order 2 and to their intersections. In particular we show that all the intersections of codimension 1 are irreducible.