I was hoping someone could help me understand the equivalence between the definitions for functions to be continuous between topological spaces, ie:(adsbygoogle = window.adsbygoogle || []).push({});

For X and Y topological spaces, and f:X-->Y a function, my notes don't prove why these definitions are equivalent (possibly because I'm missing something pretty obvious!):

1. f continuous IFF for U open in Y f^-1(U) is open in X

2. f continuous IFF for C closed in Y, f^-1(C) is closed in X

I can see why this is true when f is surjective, because then f^-1(Y) = X, so for F closed in Y, U=Y\F is open in Y and f^-1(Y) = f^-1(U u F) = f^-1(U) u f^-1(F) (because U and F are disjoint) so X = f^-1(U) u f^-1(F) implies f^-1(U) = X\f^-1(F) open in X (and then it is also easy to see the converse here). But if f is not surjective, then all that follows is that f^-1(U) is in X\f^-1(F) (open in X), so why does it follow in this case that f^-1(U) is open in X, given definition 2?

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# Help understanding equivalent definitions for continuity

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