Question #173907

Let X and Y be two topological spaces with Y Hausdorff, and let f,g : X → Y be two continuous maps. Show that: (a) If f is one-to-one, then X is Hausdorff; (b) If f(x) = g(x) ∀x ∈ A, where ¯A = X, then f = g; (c) The graph Γ = { (x,f(x)) | x ∈ X} is closed in X ×Y .


1
Expert's answer
2021-03-23T06:21:22-0400

a) We take two elements x1 and x2X,x1x2.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, UV=.=f1(UV)=f1(U)f1(V).We found two neighbourhoods f1(U) and f1(V)of x1 and x2 respectively such that f1(U)f1(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:limxx0f(x)=f(x0) (f is continuous).limxx0g(x)=g(x0) (g is continuous).f(x)=g(x) for all the interior points. So f(x0)=g(x0).a)\ \text{We take two elements } x_1\text{ and } x_2\in X, x_1\neq x_2.\\ \text{Then we find }f(x_1) \text{ and } f(x_2), f(x_1)\neq f(x_2).\\ \text{As } Y \text{ is Hausdorff there exist two neighbourhoods }U\text{ and }\\ V\text{ in Y}: f(x_1)\subset U,\ f(x_2)\subset V,\ U\cap V=\empty.\\ \empty=f^{-1}(U\cap V)=f^{-1}(U)\cap f^{-1}(V).\\ \text{We found two neighbourhoods }f^{-1}(U)\text{ and }f^{-1}(V)\\ \text{of } x_1 \text{ and } x_2\text{ respectively such that }f^{-1}(U)\cap f^{-1}(V)=\empty.\\ X\text{ is Hausdorff}.\\ b)\text{ We should show that } f(x)=g(x)\text{ on the set of all the boundary points of }X.\\ \text{Let }x_0\text{ be a boundary point of }X.\\ \text{We reach }x_0\text{ from the side of }X:\\ \lim_{x\rightarrow x_0}f(x)=f(x_0)\text{ (f is continuous)}.\\ \lim_{x\rightarrow x_0}g(x)=g(x_0)\text{ (g is continuous)}.\\ f(x)=g(x)\text{ for all the interior points. So }f(x_0)=g(x_0).

c) We take a point (x,y)Γ.(x,y)\notin \Gamma. Because yf(x)y\neq f(x) and YY is Hausdorff, there exist disjoint open sets V, VV,\ V^{\prime} with f(x)Vf(x)\in V and yVy\in V^{\prime}. ff is continuous, so U=f1(V)U=f^{-1}(V) is open with xUx\in U. We see that U×VU\times V^{\prime} is an open neighborhood of (x,y)(x,y) that does not intersect the graph. Indeed, if we suppose (for contradiction) that (z,f(z))U×V(z,f(z))\in U\times V^{\prime}, then it would follow that f(z)Vf(z)\in V^{\prime}. However, zU=f1(V)z\in U=f^{-1}(V) which means that f(z)Vf(z)\in V. Thus, f(z)VVf(z)∈V\cap V^{\prime} contradicting the fact that V, VV,\ V^{\prime} are disjoint.


Need a fast expert's response?

Submit order

and get a quick answer at the best price

for any assignment or question with DETAILED EXPLANATIONS!

Comments

No comments. Be the first!
LATEST TUTORIALS
APPROVED BY CLIENTS