Cohomology and extensions of Novikov algebras of truncated polynomials

Abstract

Let $\kk$ be a \emph{field of characteristic $p>0$, not assumed algebraically closed}, and let $V=\kk[x]/(x^p)$ be the Novikov algebra with product $a\circ b=ab'$. For $\lambda\in\kk$, let $M(\lambda)$ be Xu's module. We compute the second cohomology $\Ht(V,M(\lambda))$ for all $\lambda$ and $p$, and describe the associated abelian extensions. The computation utilizes a $\Zp$-graded presentation $V\cong\kk[t]/(t^p-1)$ to bypass truncation issues and simplify cocycle identities. We determine the exact dimensions of $\Ht(V,M(\lambda))$, showing it is $0$ for $\lambda\notin\Fp$, $3$ for $\lambda\in\Fp$ with odd $p$, and $4$ for $\lambda\in\Fp$ with $p=2$. Explicit cocycle representatives are provided for all cases. As corollaries, we show that every abelian extension of $V$ by $M(\lambda)$ splits when $\lambda\notin\Fp$. We also treat the characteristic-$0$ analogue $P=\kk[t]$: Xu's parameter $\lambda$ collapses to the single value $\lambda=0$, and $\Ht(P,M(\lambda))=0$ throughout, so $P$ is rigid (in fact formally rigid) while its positive-characteristic truncation never is --- within this family, it is truncation rather than positive characteristic per se that destroys rigidity.