Let D is a subset of R2 ,(x0,y0) element of D and let f: D to R be any function, prove that the following are equivalent.
1. f is continuous at (x0,y0).
2. For every epsilon > 0 ,there is a delta >0 such that |f(x,y) -f(x0,y0)| epsilon for all (x,y) element. Of D union Sdelta (x0,y0)
3. For every open subset V of R containing f(x0,y0) there is an open subset U of R2 contain (x0,y0) such that f(U union D) subset of V
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