a) We take two elements x1 and x2∈X,x1=x2.Then we find f(x1) and f(x2),f(x1)=f(x2).As Y is Hausdorff there exist two neighbourhoods U and V in Y:f(x1)⊂U, f(x2)⊂V, U∩V=∅.∅=f−1(U∩V)=f−1(U)∩f−1(V).We found two neighbourhoods f−1(U) and f−1(V)of x1 and x2 respectively such that f−1(U)∩f−1(V)=∅.X is Hausdorff.b) We should show that f(x)=g(x) on the set of all the boundary points of X.Let x0 be a boundary point of X.We reach x0 from the side of X:limx→x0f(x)=f(x0) (f is continuous).limx→x0g(x)=g(x0) (g is continuous).f(x)=g(x) for all the interior points. So f(x0)=g(x0).
c) We take a point (x,y)∈/Γ. Because y=f(x) and Y is Hausdorff, there exist disjoint open sets V, V′ with f(x)∈V and y∈V′. f is continuous, so U=f−1(V) is open with x∈U. We see that U×V′ is an open neighborhood of (x,y) that does not intersect the graph. Indeed, if we suppose (for contradiction) that (z,f(z))∈U×V′, then it would follow that f(z)∈V′. However, z∈U=f−1(V) which means that f(z)∈V. Thus, f(z)∈V∩V′ contradicting the fact that V, V′ are disjoint.
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