On extensions of hyperplanes of dual polar spaces

摘要

Let δ be a thick dual polar space and F a convex subspace of diameter at least 2 of δ. Every hyperplane G of the subgeometry Fsp? of δ induced on F will give rise to a hyperplane H of δ, the so-called extension of G. We show that F and G are in some sense uniquely determined by H. We also consider the following problem: if e is a full projective embedding of δ and if eF is the full embedding of Fsp? induced by e, does the fact that G arises from the embedding eF imply that H arises from the embedding eδ We will study this problem in the cases that e is an absolutely universal embedding, a minimal full polarized embedding or a Grassmann embedding of a symplectic dual polar space. Our study will allow us to prove that if e is absolutely universal, then also eF is absolutely universal.