The exact sequence of cochain complex
0→Ω∗(M)i→Ω∗(U)⊕Ω∗(V)j→Ω∗(U∩V)→0
for some open cover U,V of manifold M, yields a Long exact sequence in cohomology called 'Mayer-Vietoris' sequence.
⋯Hk−1(U∩V)d∗→Hk(M)i∗→Hk(U)⊕Hk(v)j∗→Hk(U∩V)d∗→Hk+1(M)⋯
In this complex, i∗,j∗ are induced from i,j. Since Hk is quotient, the elements of Hk are in cohomoloous classes. If we take a representative element σ∈Ω∗(M), then the map i sends this element to iσ. Based on this, one can define i∗ as follows:
i∗([σ])=([iσ])=([i∗Uσ],[i∗Vσ])∈Hk(U)⊕Hk(V)
Do similar thing to j∗, that is drop in equivalence classes instead of differential forms directly.
j∗([ω],[τ])=([j∗Vτ−j∗Uω])∈Hk(U∩V)
To make all this work, we need a connecting homomorphism map d∗ defined as follows:
d∗[η]=[α]∈Hk+1(M)
Since for k≤−1, Ωk(M)=0, the sequence can be written as,
0→H0(M)→H0(U)⊕H0(v)→H0(U∩V)→H0(M)→⋯
For a connected manifold, above sequence is exact.
0→H0(M)→H0(U)⊕H0(v)→H0(U∩V)→H0(M)→0
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