Suppose X is a pathconnected, locally compact, Hausdorff space and Y is its onepoint compactification. Let G be the fundamental group of X and H be the fundamental group of Y. Is it true that the embedding of X into Y always induces a monomorphism G>H? More generally, what is the relationship between G and H?
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More generally, if A and X_{0} are any finite CW complexes, and f : A → X_{0} is any map, let Y be the mapping cone of f, and let X be Y with the cone point removed; then Y is the onepoint compactification of X, and the inclusion X_{0} → X is a homotopy equivalence. (David's example is the case A = X_{0} = S^{1}, f = id.) So any map X → Y which is the mapping cone of something is homotopy equivalent to a onepoint compactification. I don't think you can realize any map of groups as the induced map on π_{1} of a mapping cone (0 → Z/2Z?) but you can realize (G → 0, 0 → a free group, ...) 


It is not always a monomorphism. Let X be R^2 \setminus { (x,y) : x^2 + y^2 < 1 }. Then X has fundamental group the integers, but Y is contractible. 

