A note on cohomological vanishing theorems

Abstract

We study $cd(M,N):=\sup\{j:H^j_{m}(M,N)\neq0\}$, and we prove the following over $AB$-rings: $cd(M,N)<\infty$ iff $cd(M, N)\leq2 dim R$. For locally free over the punctured spectrum, we present the better bound, namely $cd(M, N)<\infty$ iff $cd(M, N)\leq dim R,$ and show this is sharp for maximal Cohen-Macaulay, and prove that this detects freeness of $M$. We present some explicit examples to compute $cd(M, N)$. Now, suppose $R$ is only Cohen-Macaulay and of prime characteristic equipped with the Frobenius map $\varphi$. We show for some $n\gg 0$ that $cd(^{\varphi_n}R,M)<\infty$ iff $id_R(M)<\infty.$ This presents some criteria on regularity. Also, some vanishing results on $Ext^i_R(^{\varphi}R,-)$ are given, where $(-)\in\{R,^{\varphi}R\}$. We determine conditions under which the vanishing $Ext^i_R(^{\varphi}R,-)$ of restricted many $i$-th, implies the vanishing of all.