The Geometries of Jordan nets and Jordan webs

Abstract

A Jordan net (resp. web) is an embedding of a unital Jordan algebra of dimension $3$ (resp. $4$) into the space ${\mathbb{S}}^n$ of symmetric $n\times n$ matrices. We study the geometries of Jordan nets and webs: we classify the congruence-orbits of Jordan nets (resp. webs) in ${\mathbb{S}}^n$ for $n\le 7$ (resp. $n\le 5$), we find degenerations between these orbits and list obstructions to the existence of such degenerations. For Jordan nets in ${\mathbb{S}}^n$ for $n\le 5$, these obstructions show that our list of degenerations is complete. For $n=6$, the existence of one degeneration is still undetermined. To explore further, we used an algorithm that indicates numerically whether a degeneration between two orbits exists. We verified this algorithm using all known degenerations and obstructions, and then used it to compute the degenerations between Jordan nets in ${\mathbb{S}}^7$ and Jordan webs in ${\mathbb{S}}^n$ for $n=4,5$.

38 pages