Affine subspaces of antisymmetric matrices with constant rank

Abstract

For every $n \in \mathbb{N}$ and every field $K$, let $A(n,K)$ be the vector space of the antisymmetric $(n \times n)$-matrices over $K$. We say that an affine subspace $S$ of $A(n,K)$ has constant rank $r$ if every matrix of $S$ has rank $r$. Define $${\cal A}_{antisym}^K(n;r)= \{ S \;| \; S \; \mbox{\rm affine subspace of $A(n,K)$ of constant rank } r\}$$ $$a_{antisym}^K(n;r) = \max \{\dim S \mid S \in {\cal A}_{antisym}^K(n;r) \}.$$ In this paper we prove the following formulas: for $n \geq 2r +2 $ $$a_{antisym}^{\mathbb{R}}( n; 2r) = (n-r-1) r ;$$ for $n=2r$ $$a_{antisym}^{\mathbb{R}}( n; 2r) =r(r-1) ;$$ for $n=2r+1$ $$a_{antisym}^{\mathbb{R}}( n; 2r) = r(r+1) .$$