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31/01/03 MA3056: Exercise Sheet 2 — Topological Spaces 1. List all possible topologies on (i) {a, b} (ii) {a, b, c}. Consequently find a two topologies T1 and T2 on a set that are not comparable. That is, we have neither T1 ⊂ T2 nor T2 ⊂ T1 . 2. Give an example of subsets A and B of R for which A ∩ B, A ∩ B, A ∩ B and A ∩ B are all different. T 3. Let {Ti }i∈I be a family S of topologies on a set X. Show that i∈I Ti is a topology on X, but that i∈I Ti need not be. Consequently, given a topological space (Y, S) and a map f : X → Y , show that there is a weakest topology on X for which this map is continuous. Describe this topology when we take X = Y = R, S to be the usual topology, and take f to be (i) a constant function, (ii) the function that maps (−∞, 0] to 0 and (0, ∞) to 1 (iii) f (x) = x. 4. Prove that any map f : X → Y is continuous if either X is equipped with the discrete topology or Y is equipped with the indiscrete topology. 5. Show that f : X → Y is continuous if and only if it is continuous as a map onto the subspace f (X). 6. Give an example of topological spaces X and Y , a map f : X → Y and an open set U ⊂ X such that f (U ) is not open in X. 7. Let (X, T) be a topological space and let U be an open subset of X. Show that if V is a subset of U that is open in the subspace topology TU , then V is open as a subset of X. Show that this can fail if we do not assume that U is open. 8. Let X1 be a topological space, and X2 a subset of X1 equipped with the subspace topology. Let A be a subset of X2 , and denote by Ai the closure of A in Xi , for i = 1, 2. Prove that (i) A2 = X2 ∩ A1 (ii) if X2 is closed in X1 then A1 = A2 . 9. Let X1 , X2 , Y1 and Y2 be topological spaces, and let f1 : X1 → Y1 and f2 : X2 → Y2 be maps. Define a map f : X1 × X2 → Y1 × Y2 by f (x1 , x2 ) = f1 (x1 ), f2 (x2 ) . Show that f is continuous if and only if f1 and f2 are continuous, where X1 ×X2 and Y1 × Y2 are given their respective product topologies. 10. Let X and Y be topological spaces and suppose that E ⊂ X and F ⊂ Y are closed. Show that E × F is closed in the topological product X × Y . 11. (a) Let X1 and X2 be topological spaces, and let W be an open subset of X1 × X2 . Show that pi (W ) is an open subset of Xi for i = 1, 2, where pi is the projection map X1 × X2 → Xi . (b) Give an example of a closed subset W ⊂ R × R such that p1 (W ) is not closed in R. 12. Let X = [−1, 1] equipped with the usual topology. (a) Let f : X → [0, 1] be the function f (x) = |x|. Show that quotient topology induced on [0, 1] by f coincides with the usual topology. (b) Find a surjection g : X → [0, 1] for which the quotient topology induced by g is not Hausdorff. 13. Prove that homeomorphism defines an equivalence relation on the class of all topological spaces. 14. Show that if f : X → Y is a homeomorphism and A ⊂ X, then g : A → f (A) and h : X \ A → Y \ f (A) are both homeomorphisms. 15. Let X1 and X2 be topological spaces, and pick a2 ∈ X2 . Show that X1 is homeomorphic to the subspace X1 × {a2 } of X1 × X2 respectively. Show that X1 is homeomorphic to the subset {(x, x) : x ∈ X1 } of X1 × X1 . 16. Let X be an infinite set, and let T be the Zariski topology, that is, T = {∅} ∪ {U ⊂ X : X \ U is finite}. Show that T is indeed a topology, but that it is not Hausdorff. 17. Prove the following: (i) Any subspace of a Hausdorff space is Hausdorff. (ii) The product of two Hausdorff spaces is Hausdorff. (iii) If f : X → Y is continuous and injective, and Y Hausdorff, then so is X. (iv) Being Hausdorff is a topological property, that is, it is preserved by homeomorphisms. 18. Show that in a Hausdorff space X, the set {x} is (i) closed, (ii) the intersection of all open sets containing x. 19. Let f, g : X → Y be continuous maps between topological spaces, with Y Hausdorff. Show that W = {x ∈ X : f (x) = g(x)} is closed in X. Deduce that if f : X → X is a continuous map and X is Hausdorff then the fixed point set {x ∈ X : f (x) = x} is closed. 20. A T1 -space is a topological space (X, T) that satisfies the following: for any pair of distinct points x, y ∈ X there are Ux , Uy ∈ T such that x ∈ Ux , y ∈ Uy , but x ∈ / Uy , y ∈ / Ux . Show that every Hausdorff space is a T1 -space, but that there are T1 -spaces that are not Hausdorff.