# Another implememntation of van Kampen thoerem.

1. Sep 27, 2008

### MathematicalPhysicist

There this question that someone gave me an answer to, in the ask topology forum, but I feel it's an immediate conclusion of this theorem mentioned in the title, it goes like this:

Let X=X1UX2 and X1,X2 are open and simply connected in X, show that if the intersection of both sets X1 and X2 is composed of two path components, then $$\pi_1(X)$$ is isomorphic to Z the additive group of integers.

Now I think, that what I need to find is that $$\pi_1(X1)$$ and $$\pi_1(X2)$$ are isomorphic to $$\pi_1(S^1)$$ which is isomorphic to Z, and then just use the above theorem, the question which isomorphism will do the job?

any hints?

If $$X_1,X_2$$ are simply-connected, then by definition $$\pi_1(X_1) = \pi_1(X_2) = 0$$ which certainly will cause difficulties if you're trying to prove they are isomorphic to Z.