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Corollary 38.4 (Mayer Vietoris):

If $ \{U, V\}$ are open subsets of a topological space,

$\displaystyle \begin{CD} @> >> H_n(U\cap V) @> (\kappa^{\prime}, -\kappa^{\pri... ... H_n(V) @> q_n >> H_n(U\cup V) @>{\delta_n} >> H_{n-1}(U\cap V) @> >> \end{CD}$

is an exact sequence.
nisha 2012-03-20