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MATH 645, HOMEWORK #2
DUE MONDAY, MAY 29 AT 9AM. Complete the exercise 0. and 5 of the remaining 6 exercises for full credit. 0. (Required) Let X be a topological space, and let A ⊂ X. (a.) Prove that the closure of A in X, A, is the smallest closed subset of X that contains A. I.e., prove that (i) A is closed, (ii) A ⊂ A, and (iii) if C ⊂ X is any closed subset having A ⊂ C, then A ⊂ C. (b.) Prove that the interior of A in X, IntA, is the largest open subset of X that is contained within A. I.e., prove that (i) IntA is open, (ii) IntA ⊂ A, and (iii) if O ⊂ X is any open subset having O ⊂ A, then O ⊂ IntA. 1. Prove that the order topology on R×R with respect to the dictionary order agrees with the product topology Rd ×Rstd where Rd denotes R with the discrete topology and Rstd denotes R with its standard topology. 2. Let L be a straight line in the Euclidean plane. (a.) Describe the subspace topology on L when viewed as a subset of R`×Rstd where R` and Rstd are as in problem 1. (b.) Describe the subspace topology on L when viewed as a subset of R` × R`. Hint: Consider cases: (i) L is horizontal; (ii) L is vertical; (iii) L has positive slope; and (iv) L has negative slope. 3. Prove that a topological space X is Hausdorff if and only if ∆ = {(x, x) |x ∈ X} is a closed subset of X ×X in the product topology. 4. Let A be a subset of a topological space X. Define the boundary of A in X to be BdA = A ∩ (X \ A). a. Prove that x ∈ BdA if and only if every neighborhood of x intersects both A and X \A. b. Prove that A is the disjoint union of IntA and BdA. That is, show that A = IntA∪BdA and IntA ∩ BdA = ∅. c. Determine the boundary in R (with the standard topology) of the following subsets: A1 = (0, 1], A2 = {1/n |n ∈ Z+}, and A3 = Q. 5. Determine the closure of the set K = {1/n |n ∈ Z+} in R with respect to each of the following topologies: a. the standard topology b. the lower limit topology c. the K-topology d. the finite complement topology e. the discrete topology 6. Show that if A is closed in X and B is closed in Y , then A×B is closed in X × Y with the product topology. 1 2 DUE MONDAY, MAY 29 AT 9AM. Suggested Additional Problems (do not turn in): Section 16: #3, 4, 6; Section 17: #6, 11, 20