Definition 111.41.2. (Definition of delta.) Suppose that

$0 \to {\mathcal F}_1 \to {\mathcal F}_2 \to {\mathcal F}_3 \to 0$

is a short exact sequence of abelian sheaves on any topological space $X$. The boundary map $\delta : H^0(X, {\mathcal F}_3) \to {\check H}^1(X, {\mathcal F}_1)$ is defined as follows. Take an element $\tau \in H^0(X, {\mathcal F}_3)$. Choose an open covering ${\mathcal U} : X = \bigcup _{i\in I} U_ i$ such that for each $i$ there exists a section $\tilde\tau _ i \in {\mathcal F}_2$ lifting the restriction of $\tau$ to $U_ i$. Then consider the assignment

$(i_0, i_1) \longmapsto \tilde\tau _{i_0}|_{U_{i_0i_1}} - \tilde\tau _{i_1}|_{U_{i_0i_1}}.$

This is clearly a 1-coboundary in the Čech complex ${\check C}^\ast ({\mathcal U}, {\mathcal F}_2)$. But we observe that (thinking of ${\mathcal F}_1$ as a subsheaf of ${\mathcal F}_2$) the RHS always is a section of ${\mathcal F}_1$ over $U_{i_0i_1}$. Hence we see that the assignment defines a 1-cochain in the complex ${\check C}^\ast ({\mathcal U}, {\mathcal F}_2)$. The cohomology class of this 1-cochain is by definition $\delta (\tau )$.

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