Remark 3.4.7.4 (The Mayer-Vietoris Sequence). Let $X$ be a topological space, let $U,V \subseteq X$ be subsets whose interiors $\mathring {U} \subseteq U$ and $\mathring {V} \subseteq V$ comprise an open covering of $X$, and set $K = \operatorname{Sing}_{\bullet }(U) \coprod _{ \operatorname{Sing}_{\bullet }(U \cap V) } \operatorname{Sing}_{\bullet }(V)$. Then the inclusion $K \hookrightarrow \operatorname{Sing}_{\bullet }(X)$ induces a quasi-isomorphism $\mathrm{C}_{\ast }(K; \operatorname{\mathbf{Z}}) \hookrightarrow \mathrm{C}_{\ast }(X; \operatorname{\mathbf{Z}})$ (by virtue of Theorem 3.4.6.1 and Proposition 3.1.6.18), and we have a short exact sequence of chain complexes
Passing to homology groups (see Construction 
), we obtain a long exact sequence of abelian groups
which we refer to as the Mayer-Vietoris sequence of the covering $\{ U,V \} $. The existence of this sequence is essentially equivalent to the statement of Theorem 3.4.7.3.