Leray's theorem

In algebraic topology and algebraic geometry, Leray's theorem (so named after Jean Leray) relates abstract sheaf cohomology with Čech cohomology.

Let ${\displaystyle {\mathcal {F}}}$ be a sheaf on a topological space ${\displaystyle X}$ and ${\displaystyle {\mathcal {U}}}$ an open cover of ${\displaystyle X.}$ If ${\displaystyle {\mathcal {F}}}$ is acyclic on every finite intersection of elements of ${\displaystyle {\mathcal {U}}}$, then

${\displaystyle {\check {H}}^{q}({\mathcal {U}},{\mathcal {F}})={\check {H}}^{q}(X,{\mathcal {F}}),}$

where ${\displaystyle {\check {H}}^{q}({\mathcal {U}},{\mathcal {F}})}$ is the ${\displaystyle q}$-th Čech cohomology group of ${\displaystyle {\mathcal {F}}}$ with respect to the open cover ${\displaystyle {\mathcal {U}}.}$