# 1 Topology Definition of a topology Basis (Base) of a topology The subspace topology & the product topology on X Y 3

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1 Index Page 1 Topology Definition of a topology Basis (Base) of a topology The subspace topology & the product topology on X Y Basic topology concepts: limit points, closed sets, and more Metric topology Continuous functions Compactness Countability axioms Separation axioms 11 1

2 1 Topology 1.1 Definition of a topology Definition A topology T on a set X is a subset of 2 X which satisfies: (1), X T. (2) If U α T for all α I, then α I U α T. (3) If U 1,, U n T, then 1 i n U i T. X equipped with some topology T is called a topological space. Each U T is called an open set (for topology T ). Remark Think about the definition of open sets in R n. Definition For any two different topologies T and T on X, if T T, then we say T is weaker or coarser than T, and T is finer than T. 1.2 Basis (Base) of a topology Definition A basis (or base) B for X is a subset of 2 X which satisfies: (1) For any x X, there exists B B so that x B. (2) For any x B 1 B 2, where B 1, B 2 B, there exists B 3 B so that x B 3 and B 3 B 1 B 2. Theorem T := {U X : for any x U, there exists B B s.t. x B U} is a topology. Definition The topology defined in Theorem is called the topology generated by basis B. Remark Think about the set of all open balls in R n. Theorem The topology T generated by basis B equals the collection of all unions of elements of B. Proof. If U belongs to the topology T generated by basis B, then for any x U, there exists B x B so that x B x U, and we may write U = x U B x. Conversely, if U = α B α, then for each x U, x B α U for some α. 2

3 We may think of basis as building blocks of a topology. Sometimes it may not be easy to describe all open sets of a topology, but it is often much easier to find a basis for a topology. Also notice that a topology may be generated by different bases. Theorem Let B, B be bases for T, T, respectively. Then T T if and only if for any B B, x B, we can find B B s.t. x B B. Proof. (Only if) Pick B B, and arbitrary x B. Since B T T, we may find B B so that x B B. (If) Let U T. For any x U, we may find B B so that x B, and B B s.t. x B B U. Therefore, U = x U B x, where each B x B, which shows U T. 1.3 The subspace topology & the product topology on X Y Definition Let Y X and T be a topology of X. The subspace topology T Y Y (induced from T ) is defined as T Y := {Y U : U T }. of Remark Think about the unit circle with center 0 in R n. Exercise Find a basis for T Y? Definition Let T 1, T 2 be two topologies of X, Y, respectively. The product topology T 1 T 2 on X Y is the topology generated by the basis B := {U V : U T 1, V T 2 }. Remark We may define product topology of X 1 X n similarly. However, things are more complicated when we try to define the product topology on infinitely many (countably or uncountably) product spaces. May see Section 19 of [Munkres]. 1.4 Basic topology concepts: limit points, closed sets, and more Definition A subset A of X is closed if X \ A is open. 3

4 Exercise Show that, X are both closed. Show that after taking finitely many union operations or arbitrary intersections of closed sets, we still have closed sets. Definition Let A X. The interior of A, denoted by int(a), is the union of all open sets contained in A. The closure of A, denoted by A, is the intersection of all closed sets containing A. Theorem A is open if and only if A = int(a); A is closed if and only if A = A. Proof. If A is open, then the union of all open sets contained in A includes itself. If A = int(a), then A is open by the definition of a topology. The second half is left to the reader. Definition Let A be a subset of the topological space X with topology T, and x X. We say x is a limit point of A if for any U T and x U, (U \ {x}) A. Remark Think about the unit circle centered at 0 in R n, and the region enclosed by it. Theorem Let A be the set of all limit points of A. We have A = A A. Proof. If x A, then x A. If x A, then x / U for any U T s.t. A U c (:= X \ U) (Otherwise, (U \ {x}) A =, a contradiction). This shows x U c for any U T s.t. A U c, that is, x A. Conversely, If x / A A, then for some U T and x U, U A = ({x} A) ((U \ {x}) A) =. Therefore, x / U c, where A U c, and hence x / A. Corollary A closed set consists of all its limit points. Proof. The result follows from Theorem and Theorem Theorem For any subset A, B of a topological space X, A B = A B. Proof. A A B A B = A B. Definition Let x, x 1, x 2,, x n, X. We say x n converges to x, denoted by x n x, if for every U T s.t. x U, there exists N N so that for all n > N, we have x n U. Exercise Show that if x n x, where x n A and x n x for all n N, then x is a limit point of A. Conversely, If x is a limit point of A, then there exists a sequence 4

5 {x n } A so that x n x. Example (Multiple limits) Let X = {1, 2, 3}, T = {, X, {1, 2}, {2, 3}, {2}}. What is the limit of the sequence {2, 2,, 2, }? We now introduce a new topology concept to make sure we always have unique limit points. Definition (Hausdorff space) A topological space X with topology T is called a Hausdorff space if for any x y, x, y X, we can find U, V T so that x U, y V, and U V =. Theorem If X is a Hausdorff space, then any sequence of points of X converges to at most one point in X. Proof. Straightforward. (What happens if the sequence converges to two distinct points?) Lemma Let X be Hausdorff. Then for every x X, {x} is a closed set. Proof. Try to write X \ {x} as a union of open sets. Theorem Let X be a Hausdorff space, and A X. Then x X is a limit point of A if and only if every neighborhood of x contains infinitely many points of A. (Neighborhood of x: open set containing x) Proof. If part is trivial. For only if part, assume that for some neighborhood U of x, there re only finitely many points of A in U, say, x 1,, x N. By Lemma , U \ {x 1,, x N } is an open set. We have ((U \ {x 1,, x N }) \ {x}) A =, which shows that x is not a limit point of A. Remark The assumptions made in Theorem can be weakened (It suffices to assume that X is a T 1 space, see Definition 1.9.1). See also [Munkres]. Theorem Let T and T are two distinct topologies for X s.t. T T. If x n x under T, then x n x under T. Remark Theorem shows it is easier for a sequence to converge in a weaker topology. We would say the convergence is weaker in weaker topology. 5

6 1.5 Metric topology Definition A metric on a set X is a function d : X X R so that (1) d(x, y) > 0 for all x y, d(x, x) = 0. (2) d(x, y) = d(y, x). (3) d(x, y) + d(y, z) d(x, z). Definition A topological space X with topology T is called a metric space if T is generated by the collection of balls (which forms a basis) B(x, ɛ) := {y : d(x, y) < ɛ}, x X, ɛ > 0. Whenever there is a metric d s.t. T is generated this way, we say X is metrizable. Definition Let (X, d) be a metric space. We say A X is said to be bounded if there exists M > 0 s.t. d(a, b) < M for all a, b A. Definition A metric space (X, d) is said to be totally bounded if there exists finitely many balls of the same radius that covers X. Definition Let (X, d) be a metric space. A Cauchy sequence is a sequence {x n } X s.t. for every ɛ > 0, there exists N N, we have d(x n, x m ) < ɛ for all n, m > N. A metric space X is called a complete metric space if every Cauchy sequence in X converges to some point in X. 1.6 Continuous functions Definition Let X, Y be topological spaces. A function f : X Y is said to be continuous if for any U open in Y, f 1 (U) is open in X. Theorem Let X, Y be topological spaces, and f : X Y, then TFAE: (1) f is continuous. (2) For any U closed in Y, f 1 (U) is closed in X. (3) For every x X and each neighborhood V of f(x), there exists a neighborhood U of x s.t. f(u) V. If, furthermore, both X and Y are metric spaces, then all the three statements are equivalent to the following one: (4) For every x X and ɛ > 0, there exists δ > 0 s.t. f(b(x, δ)) B(f(x), ɛ). Proof. We ll prove the equivalence of (1),(2), and (3), and the rest are left as exercises. First of all, (1) (2) is straightforward. For (1) (3), we may simply let U = f 1 (V ), and we can 6

7 easily find x U and U is open. For (3) (1), for any V open in Y, and x X, we may find a neighborhood U x for x s.t. f(u x ) V, and this implies x U x f 1 (f(u x )) f 1 (V ). Therefore, f 1 (V ) equals the union of all these U x s, and thus f 1 (V ) is open in X. Definition Let X, Y be topological spaces. A function f : X Y is called a homeomorphism if both f and f 1 are continuous. If such homeomorphism exists between X and Y, then we say X is homeomorphic to Y. Remark All topological properties are preserved by homeomorphisms. We may see homeomorphism as an equivalence relation between topological spaces. 1.7 Compactness Definition A subset A of a topological space X is said to be compact if every open covering of A contains a finite subcover. Theorem Every closed subset C of a compact set D is compact. Proof. Let A be an open covering of C. We have that {A, C c } is an open covering of D. By compactness of D, we can find a finite subcover {U 1,, U n, C c } that covers D. It turns out that {U 1,, U n } covers C. Theorem Any finite union of compact sets is compact. Proof. It suffices to show that if C and D are compacts, then so is C D. Let A = α I U α be an open covering of C D. Therefore, there exists N j=1 U n j and M j=1 U m j that covers C, D, respectively. N j=1 U n j M j=1 U m j is therefore a finite subcover of C D. Theorem In a Hausdorff space X, every compact subset D is closed. Proof. Fix x X \ D. For any y D, we may find some neighborhood U y of y and V y,x of x so that U y V y,x = (Since X is Hausdorff). Since D is compact, we may find finitely many such U y s that cover D, Say U y1,, U yn. Since U y1 U yn and V y1,x V yn,x are disjoint, it follows that V y1,x V yn,x is a neighborhood of x which lies in X \ D. This proves that X \ D is open, and thus D is closed. Remark Theorem is false if the underlying space is not Hausdorff. Again, consider X = {1, 2, 3}, T = {, X, {1, 2}, {2, 3}, {2}}. Every subset of X is a compact set, but not every subset of X is a closed set. Corollary In a Hausdorff space X, arbitrary intersection of compact sets is still compact. 7

8 Proof. By theorem 1.7.4, arbitrary intersection of compact sets is closed. By Theorem 1.7.2, it is compact. Theorem Continuous functions map compact sets to compact sets. Proof. Let f : X Y be a continuous function, X, Y are topological spaces. Let A be a compact set in X. For any open covering {U α } of f(a), {f 1 (U α )} is an open covering of f 1 (f(a)) and hence of A. Compactness of A implies that A n j=1 f 1 (U j ). Therefore, f(a) f( n j=1 f 1 (U j )) = n j=1 f(f 1 (U j )) = n j=1 U j, which shows f(a) has a finite subcover of {U α }. Exercise Show that D is compact if and only if for any collection C of closed subsets of D s.t. the intersection of any finite subcollection of C is nonempty, the intersection of all elements in C is nonempty. Definition A subset D of a topological space X is said to be limit point compact if every infinite subset A of D, there exists a point x D so that x is a limit point of A. Definition A subset D of a topological space X is said to be sequentially compact if every sequence of D contains a convergent subsequence which converges to some point x in D (x does not necessarily belong to the sequence). Theorem If a subset D of a topological space X is sequentially compact, then it is limit point compact. Proof. Let A be an infinite subset of D. We may pick countably many of them, and list them as a subsequence {x n }. By the assumption we have x n x D, so every neighborhood U x of x must contain a point other than x. This shows (U x \ {x}) A. Theorem If a subset D of a topological space X is compact, then it is limit point compact. Proof. Let A be an infinite subset of D. If there is no x D s.t. x is a limit point of A, then for each x A D, we may find some neighborhood U x of x so that U x A = {x}. All these U x s form an open covering of D, but it is easily seen that there is no finite subcover of this open covering. Therefore, there must be some x D s.t. x is a limit point of A, and this proves D is limit point compact. Remark In general topological spaces, we do have examples that neither of the converse propositions of Theorem and Theorem is true. Theorem The following statements are equivalent for any subset D of the metric space (X, d): 8

9 (1) D is compact. (2) D is complete and totally bounded. (3) D is limit point compact. (4) D is sequentially compact. Proof. For (1) (2), (2) (3), see [Dudley]. For (3) (4), (4) (1), see [Munkres]. Remark Recall Heine-Borel theorem: In R n, a set is a compact set if and only if it is closed and bounded. 1.8 Countability axioms Definition A topological space X is called first countable if X has a countable basis at each x X, which means there exists a basis B that generates the topology, and there exists a countable subcollection of B so that x falls in each of them, and every neighborhood of x contains at least one basis element of this countable subcollection. Definition A topological space X with topology T is called second countable if there exists a countable basis that generates T. Theorem second countability of a topological space implies first countability Definition A subset A is dense in a topological space X if A = X. Definition A topological space X is said to be separable if there exists A s.t. A = X, and A has only countably many elements. Theorem A subset A is dense in a topological space X if and only if for every x X and every open set U x containing x, U x contains at least one element in A. Proof. By Theorem 1.4.7, we have A = A A. If A is dense in X, then every x is either in A or is a limit point of A, and in either case it is easily seen that the second assertion of the theorem is true. Conversely, for any x X, if every neighborhood U x of x satisfies (U x \{x}) A, then x A. If there is some neighborhood U x of x s.t. (U x \{x}) A =, then x A (Otherwise we would have U x A =, which contradicts our assumption). Therefore, x A A = A. Theorem If a topological space X is second countable, then it is separable. Proof. Let B = {B 1, B 2, } be a countable basis for X. For each B j, we may pick some x j B j. The set {x 1, x 2, } is dense in X. 9

10 Theorem Any metric space (X, d) is first countable. Proof. For each x X, consider B(x, ɛ) = {y : d(x, y) < ɛ}, where ɛ Q +. Theorem Any separable metric space (X, d) is second countable. Proof. Let A be a countable dense subset of X. Consider the collection of B(x, ɛ) = {y : d(x, y) < ɛ}, where ɛ Q +, x A. The goal is to show that the collection of these balls form a basis, which is left as an exercise. Corollary Any subset A of a separable metric space X is also separable with the subspace topology induced from X. Proof. By the previous theorem, X has a countable basis. The subspace topology on A is thus generated by a countable basis, too. By Theorem 1.8.7, A is separable. Theorem Any compact metric space (X, d) is second countable. Proof. For every n N, we may cover X with finitely many balls B(x, 1/n), say B(x n,1, 1/n),, B(x n,k(n), 1/n). The collection of all these balls form a countable basis. (Details are left to the readers.) The next theorem is an application of first countability. Theorem Let X, Y be topological spaces. If the function f is continuous, then for any convergent sequence x n x in X, the sequence f(x n ) f(x) in Y. The converse is true if X is first countable. Proof. Let f be continuous. By Theorem 1.6.2, for all x X, and each neighborhood V of f(x), there exists a neighborhood U of x s.t. f(u) V. Now, for any sequence {x n } s.t. x n x, x n U for all n large, and hence f(x n ) V for all n large. This implies f(x n ) f(x). Let C be a closed set in Y and x be a limit point of f 1 (C). Since X is first countable, we may find a countable collection of basis elements {B 1, B 2, } so that for any open set U containing x, U contains at least one of these B is. Now we let B 1 = B 1, B 2 = B 1 B 2, B 3 = B 1 B 2 B 3, and so forth. Since x is a limit point of f 1 (C), we may pick some x j (B j \ {x}) C for every j N. it s not hard to see from definition that x n x. Therefore, f(x n ) f(x) (Note that f(x n ) C for all n N), and this shows f(x) C = C (By Theorem 1.4.7). Therefore, x f 1 (C), and hence f 1 (C) is closed. 10

11 1.9 Separation axioms Definitions Let X be a topological space. (1) X is said to be a T 1 space if all singletons {x} are closed. (2) X is said to be a T 2 space if it is a Hausdorff space (See Definition ). (3) A T 1 space X is said to be a T 3 space, or a regular space, if for any x X, C closed in X, x / C, we can find open sets U, V so that x U, C V, and U V =. (4) A T 1 space X is said to be a T 4 space, or a normal space, if for any C 1, C 2 closed in X, C 1 C 2 =, we can find open sets U, V so that C 1 U, C 2 V, and U V =. Remark There re also T 3 1 (completely regular) and T 5 (completely normal) topologies, which are not discussed here. Also, it is obvious that T 4 T 3 T 2 T 1 (See also 2 Lemma ). Theorem Every regular space X with a countable basis (2nd countable) is normal. Proof. See [Munkres]. Theorem Every metric space (X, d) is normal. Proof. Let C 1 and C 2 be two closed sets in X. For each x C 1, Pick a tiny ball B(x, ɛ x ) so that B(x, ɛ x ) C 2 =. Similarly, for each y C 2, Pick a tiny ball B(y, ɛ y ) so that B(y, ɛ y ) C 1 =. We claim that U 1 := x C 1 B(x, ɛ x /3) and U 2 := y C 2 B(y, ɛ y /3) are two open sets that separates C 1 and C 2. The only nontrivial thing we need to show is that U 1 U 2 =. If z U 1 U 2, then z B(x, ɛ x /3) and y B(y, ɛ y /3) for some x C 1 and y C 2. From triangle inequality we have d(x, y) < ɛ x /3 + ɛ y /3 max(2ɛ x /3, 2ɛ y /3). WLOG we assume that ɛ x = max(ɛ x, ɛ y ), and hence we have B(x, ɛ x ) C 2, but this is a contradiction to our assumption. Theorem Every compact Hausdorff space X is normal. Proof. Let C 1 and C 2 be two closed sets in X. As closed subsets of a compact set, C 1 and C 2 are both compact (Theorem 1.7.2). For each x C 1, we may find two open sets V x containing C 2 and U x containing x so that U x V x = (Why?). Cover C 1 with finitely many U x1 U xn, we find that U x1 U x2 U xn and V x1 V x2 V xn are two open sets that separates C 1 and C 2. Theorem (Urysohn metrization theorem) Every second countable regular space X is metrizable. Proof. See [Munkres]. 11

12 Now we state two famous theorems, the Stone-Weierstrass theorem and the Urysohn s lemma, before we proceed to Theorem Lemma (Stone-Weierstrass Theorem) Let X be a compact Hausdorff space, C(X, R) be the set of all real-valued continuous functions on X. Let A be a collection of functions which satisfies (1) For any a, b R, f, g A, af + bg A. (2) For any f, g A, fg A. (3) For any x, y X, we may find some f A so that f(x) f(y). (4) A contains all constant functions. Then A is dense in C(X, R) with supremum norm. Proof. See [Peter Lax], p126. Lemma (Urysohn s lemma) Let X be a normal space; let A and B be disjoint closed subsets of X. Let [a, b] be a closed interval in the real line. Then there exists a continuous map f : X [a, b] such that f(x) = a for every x A, and f(x) = b for every x B. Proof. See [Munkres]. Theorem For any compact Hausdorff space X, the following are equivalent: (1) X is second countable. (2) X is metrizable. (3) The space C(X, R) with the supremum norm is separable. Proof. (1) (2): By both Theorem and Theorem (2) (1): By Theorem For (2) (3): From the proof in Theorem , we know that a basis that generates the topology of X is given by a countable collection of balls B(x, 1/n), n N, and we list them as B := {B 1, B 2, }. For each i j N, when B i B j, we apply Lemma to define f i,j (x) = f j,i (x) = 1 if x B i, f i,j (x) = f j,i (x) = 0 if x / B j, and f i,j f j,i C(X, R). For all other cases of B i, B j we simply define f i,j 1 C(X, R). Define A to be the collection of all possible linear combinations (with real coefficients) of products of finitely many functions f i,j. It s not hard to see the conditions (1)(2) in Theorem (Stone-Weierstrass Theorem) are satisfied. For condition (3) in Theorem 1.9.7, consider x, y X, we can find some ball B m B s.t. x B m and y / B m, and there exists B n B s.t. x B n B m. We have f n,m (x) = 1 and f n,m (y) = 0. For condition (4) in Theorem 1.9.7, note that it is always possible for us to find B i, B j s.t. B i B j and B j B i, and f i,j 1. From the definition of A we know that all constant functions lie in A. Therefore, by Theorem 1.9.7, A is dense in C(X, R) with supremum norm. 12

13 Let A be the collection of all possible linear combinations (with rational coefficients) of products of finitely many functions f i,j. It s not hard to see A is dense in A with sup norm, and thus A is dense in C(X, R) with sup norm. This proves (3). For (3) (1): Let {f n : n N} be dense in C(X, R) (with sup norm). We claim that B defined to be the collection of all finite intersections of sets of form fn 1 (p, q), n N, p, q Q, is a basis. To see this, for any open set U X and x U, define f(x) = 1 and f(y) = 0 for all y / U, so that f C(X, R) by Theorem (Urysohn s lemma). We may find some N s.t. x f 1 N (0.9, 1.1) f 1 (0.8, 1.2) U. Besides, B is closed under finite intersection by its definition. Thus B is a countable basis, and (1) is proved. 13

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