I didn't see a topology forum, so I thought I'd post this question here. Can anyone give any pointers on using van Kampen's theorem? I understand the basic way it works, decompose a space X into open, path-connected sets, say U and V. Then pi1(U) * pi1(V) = pi1(X)/N, where N is a normal subgroup. The N is what gives me trouble. How do calculate what the normal subgroup is? Any help is appreciated!