Past exam of the mathematics course of the University of Cambridge / 2016 / iii / Paper 118 / 4 / a

For the usual additive Čech cohomology groups, $\mathcal{F}$ is understood to be a sheaf of abelian groups. Order the index set of the open cover $\mathfrak{U}=(U_i)$. The Čech cochain group isAn empty intersection contributes the zero group. The Čech coboundary is the alternating sum of restriction maps:Every double omission appears twice with opposite sign, so ${\delta }^2=0$. The Čech cohomology is the cohomology of this cochain complex:Here ${\check{C}}^{-1}=0$. The sheaf gluing axiom identifies ${\check{H}}^0(\mathfrak{U},\mathcal{F})$ with $\Gamma (X,\mathcal{F})$. This definition is for the fixed cover; no limit over refinements is part of the group requested here. Without the abelian-group structure, this additive Čech cochain complex is not defined.