Definition Let X be a subset of C. The closure of X is the subset \overline{X} of C defined by: \overline{X} = X \cup \{x \in C | x is a limit point of X\}. Problem 1 Show that X \subset C is closed if and only if X = \overline{X}. Problem 2 Show that the closure of X \subset C satisfies \overline{X} = \overline{\overline{X}}. Problem 3 Given any subset X \subset C, show that the closure \overline{X} is closed.
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Consider the topology T on X = {a, b, c, d, e}, where T = {X, ∅, {a}, {a, b}, {a, c, d}, {a, b, c, d}, {a, b, e}} (a). List the closed subsets of X. (b). Determine the closure of the sets {a}, {b} and {c, e}. (c). Which of the sets in (b) are dense in X? 2. Let T be the same topology as given in Problem 1. (a). Find the interior points of the subset A = {a, b, c} of X. (b). Find the exterior points of A. [ Definition: Exterior of a set A, is the interior of the complement of A, i.e. ext(A) = int(A^c)]. (c). Find the boundary points of A. 3. Let X be a topological space. A point p ∈ X is called a limit point(accumulation point) of a subset A of X iff every open set O containing p, contains a point of A different from p. .i.e. p ∈ O, where O is open implies (O {p}) ∩ A ≠ ∅. The set of all limit points of A is called the derived set of A and denoted by A'. (If X = ℝ with standard topology, then this definition is the same as Chapter 2). Let T be the same topology as problem 1. Determine the derived sets of (a). A = {c, d, e} (b). B = {b}. 4. Let A be a subset of B in a topological spaces X. Prove that a limit point of A is also a limit point of B.
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Let (X, d) be a metric space, a ∈ X and r ∈ R such that r > 0. (a) Define each of the following concepts (i) The ball with centre a and radius r (ii) A neighbourhood of a (iii) An interior point of a set S ⊆ X. (iv) An open subset of X (v) A closed subset of X (vi) Adherent points (vii) Accumulation points (viii) A dense set. (ix) The closure of a subset of X (b) Let x0 be any element of (X, d) Show that {x0} is closed (c) Let S be a subset of a metric space Show that S̄ = S ∪ S' (d) Let A and B be subsets of a metric space X (i) Prove that A° ∪ B° ⊆ (A ∪ B)° where A° and B° are the interior points of A and B respectively (ii) Give an example of two subsets A and B of the real line such that A° ∪ B° ≠ (A ∪ B)°
Problem 1. Find sets A ⊂ B in (ℝ, |∙|) such that A is open in ℝ and relatively closed in B. Problem 2. Let (x_n)_{n=1}^∞ be a sequence in a metric space (X, d). (a) Suppose (x_n)_{n=1}^∞ is a Cauchy sequence. Show that {x_n | 1 ≤ n < ∞} is a bounded set. (b) Suppose (x_n)_{n=1}^∞ is a Cauchy sequence, and that there is a subsequence (x_{n_j})_{j=1}^∞ which converges to some x ∈ X. Show that also lim_{n→∞} x_n = x. (c) We say that x ∈ X is a limit point of the sequence (x_n)_{n=1}^∞ if ∀ε > 0, ∀N ∈ ℕ, ∃n ≥ N : d(x_n, x) < ε. Show that x is a limit point of (x_n)_{n=1}^∞ if and only if there exists a subsequence (x_{n_j})_{j=1}^∞ which converges to x.
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