Each arc X,Y is diffeomorphic to an interval and thus to R.
Here instead of writing one forms and seeking existence of integral solutions, we can use Mayer-Vietoris sequence to make computations. The Mayer-Vietoris sequence for S1 is as follows:
Using dimensional formula for sequence of vector spaces ∑nk=0(−1)kdk, we can figure out dimension of H1(M)=d1=1 as follows:
Since S1 is connected, H0(S1)=R. As shown before H0(U)=H0(V)=R. Since overlaps are disjoint we have R⊕R. All this results in the following sequence.
Notice j∗:H0(U)⊕H0(V)j∗→H0(U∩V) is given as follows. Since, we are dealing with 0 dimensional space, corresponding vectors are 0 dimensionals - that is scalars or real numbers.
That is these are elements of diagonal in R×R. d∗:H0(U∩V)d∗→H1(M) sends this to one dimensional space which is isomorphic to R, hence points m≠n will result in this element. Among many, we can choose one.
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