Extensions at Nonzero p-Characters for Cartan-Type Lie Algebras

Abstract

For restricted simple Cartan-type Lie algebras over an algebraically closed field of characteristic $p\ge5$, the reduced enveloping algebra at every nonzero $p$-character is a matrix algebra over a terminal symmetric corner, and this corner determines the extensions. Central-socle traces recover its radical and its simple factors, and the first radical layer determines the degree-one extensions. For the Witt algebra $W(1;1)$, at even height $2\le h\le p-3$, the unique simple module has self-extension dimension $\min\{p-h-2,h+2\}$. At height two in characteristic seven, we present the Yoneda algebra as an eight-dimensional symmetric algebra tensored with a polynomial algebra on three central degree-two classes. An equivariant Tate resolution and cocycle identities prove the presentation in all degrees.