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Notes on Topology An annex to H104,H113, etc. Mariusz Wodzicki December 3, 2010 1 Five basic concepts open sets o / closed sets (1) O g 7 O ' w neighborhoods 7 h w ( interior o / closure 1.1 Introduction 1.1.1 A set X can be made into a topological space in five different ways, each corresponding to a certain basic concept playing the role of a primitive notion in terms of which the other four are expressed. Thus, one can talk of the topology on a set X in the language of open sets, or the language of closed sets, or the language of the interior operation, or the language of the closure operation or, finally, the language of neighborhoods of an arbitrary point p 2 X. 1 The five ‘languages’ are completely equivalent which means, in par- ticular, that one can faithfully translate from any one of them into any other one. 1.1.2 The closure operation A mapping P(X)−!P(X), E 7−! E¯ , is said to be a (topological) closure operation if it satisfies the following conditions for any E, F ⊆ X: (C1) E ⊆ E¯ ¯ (C2) E¯ = E¯ (C3) E [ F = E¯ [ F¯ (C4) Æ = Æ Exercise 1 Show that E¯ ⊆ F¯ whenever E ⊆ F. 1.1.3 The interior operation A mapping P(X)−!P(X), E 7−! E˚ , is said to be an interior operation if it satisfies the following conditions for any E, F ⊆ X: (I1) E˚ ⊆ E ˚ (I2) E˚ = E˚ ◦ (I3) (E \ F) = E˚ \ F˚ (I4) X˚ = X Exercise 2 Show that E˚ ⊆ F˚ whenever E ⊆ F. 1.1.4 If E 7−! E¯ is a closure operation on X, then c E 7−! E˚ ˜ (Ec) (2) 2 is an interior operation. And vice-versa, if E 7−! E˚ is an interior operation on X, then E 7−! E¯ ˜ ((Ec)◦)c (3) is a closure operation. Exercise 3 Prove the above statement. 1.1.5 The two operations are conjugate to each other. This is represented by the commutativity of the following diagram ◦ () P(X) P(X) u w u ()c ()c (4) u () u P(X) P(X) w The conjugating operation, E 7−! Ec , is an example of anti-automorphism of a partially ordered set: it is invertible (in fact, it is equal to its own in- verse), but reverses the order instead or preserving it: Ec ⊇ Fc if E ⊆ F.(5) Moreover, it interchanges the operations of union and intersection: (E [ F)c = Ec \ Fc and (E \ F)c = Ec [ Fc.(6) 1.1.6 Open sets A subset T ⊆ P(X) is said to be a topology on a set X if it satisfies the following conditions S (T1) for any U ⊆ T , one has U 2 T T (T2) for any finite U ⊆ T , one has U 2 T (T3) X 2 T (T4) Æ 2 T Members of T are referred to as open sets or, more precisely, open subsets of X. 3 1.1.7 Closed sets Complements of members of any topology on X form a subset Z ⊆ P(X) which possesses the following properties T (Z1) for any W ⊆ Z , one has W 2 Z S (Z2) for any finite W ⊆ Z , one has W 2 Z (Z3) Æ 2 Z (Z4) X 2 Z Members of Z are then referred to as closed sets or, more precisely, closed subsets of X. 1.1.8 Neighborhoods A family fNpgp2X , indexed by elements of X, of subsets Np ⊆ P(X) is said to be a neighborhood system on a set X if it satisfies the following conditions (N1) if M, N 2 Np , then M \ N 2 Np 0 0 (N2) if N 2 Np and N ⊆ N , then N 2 Np (N3) if N 2 Np , then fq 2 N j N 2 Nqg 2 Np T (N4) p 2 Np (N5) X 2 Np Members of Np are referred to as neighborhoods of point p. 1.2 An interlude: Filters and filter-bases 1.2.1 Directed sets A nonempty partially ordered set (L, ) is said to be directed if for any l, m 2 L there exists n 2 L such that l n and l n (7) or, equivalently, if any nonempty finite subset of I is bounded above. 4 1.2.2 Filter-bases A nonempty subset F ⊆ P(X) is said to be a filter-base on a set X if it satisfies the following conditions (F1) for any E, F 2 F , there exists G 2 F such that G ⊆ E \ F 2 F (F2) Æ < F . In other words, a filter-base on a set X is a directed subset F ⊂ P(X) which does not contain Æ, the smallest element of (P(X) ⊆). Exercise 4 Show that the intersection of finitely many members of a filter-base is nonempty: E1 \···\ En , Æ (E1,..., En 2 B). 1.2.3 Filters A filter-base that satisfies one more condition: (F3) for any subsets E ⊆ F ⊆ X, if E 2 F , then also F 2 F , is called a filter on a set X. 1.2.4 The filter generated by a filter-base For a given filter-base B ⊆ P(X) on a set X, define B∗ ˜ fF ⊆ X j F ⊇ E for some E 2 Bg (8) Exercise 5 Show that B∗ satisfies condtitions (F1)-(F3) above. Show that any filter F containing B contains B∗ as well. Thus, B∗ is the smallest filter containing B. We shall refer to it as the filter generated by B. 5 1.2.5 Example: principal filters For any nonempty subset E ⊆ X, the family PE of all subsets of X which contain E , is a filter. In fact, it is the filter generated by the filter-base consisting of one single set E: PE ˜ fEg∗.(9) We shall call such filters principal. In case, E is a singleton set fpg for some p 2 X, we may write Pp instead of Pfpg . Exercise 6 Show that any filter on a finite set X is principal. Exercise 7 Show that if F 2 F , then PF ⊆ F . Exercise 8 Show that any filter F on a set X is the union of principal filters. More precisely, [ F = PF.(10) F2F 1.2.6 Example: the Frechet´ filter of a directed set Let (S, ) be a partially ordered set. Exercise 9 Show that the image of S under the canonical emebedding of (S, rev ) into P(S), f[si j s 2 Sg,(11) is a filter-base on S if and only if (S, ) is directed. In that case, the filter generated by (11) is called the Frechet´ filter on directed set (S, ). We shall denote it Fr(S, ) or, simply, Fr(S), if the order on S is clear. Exercise 10 Show that Fr(N, ≤) consists of all subsets E ⊆ N such that the complement, Ec , is finite. 6 1.2.7 Example: the direct image of a filter Exercise 11 Let B be a filter-base on a set X and f : X−!Y be a mapping. Show that the images of all members of B under f , f (B) ˜ f f (B) j B 2 Bg,(12) form a filter-base on Y. In other words, filter-bases on X are sent by mappings f : X−!Y to filter-bases on Y. If F is a filter on X, its image, f (F ), is only a filter- base however: a subset G of Y which contains f (E) does not have to be of the form f (F) for some F 2 F . It does not even have to be of the form f (F) for any F 2 P(X). For this reason, we consider instead the generated filter f∗F ˜ ( f (F ))∗ (13) and call it the direct image of filter F under mapping f . 1.2.8 Example: the elementary filter associated with a net A net in a set X is just a family of elements of X, indexed by a certain directed set L. In other words, a net x = fxlgl2L in X is the same as a mapping x : L−!X, i 7−! xl. 1.2.9 A net indexed by the set of natural numbers, N, or by any of its subsets, is called a sequence in X. 1.2.10 The elementary filter associated with a net The direct image of the Frechet´ filter, x∗(Fr(L)) (14) is called the elementary filter associated with net x . 7 1.2.11 Example: the inverse image of a filter In contrast with the image of a filter-base, the preimage of a filter-base is generally not a filter-base, it is not even contained in any filter, and the reason is clear: the preimage of a nonempty subset may be empty. Exercise 12 Let C be a filter-base on a set Y and f : X−!Y be a mapping. Show that the preimages of all members of C under f , f −1(C ) ˜ f f −1(C) j C 2 C g,(15) form a filter-base on X if and only if C \ f (X) , Æ for every member C 2 C . (16) 1.2.12 Let G be a filter on Y. If it satisfies condition (16), then the filter generated by f −1(G ), ∗ ˜ −1 f (G ) f (G ) ∗ (17) is called the inverse image of filter G . 1.3 The neighborhood filters 1.3.1 Let us return to the definition of a system of neighborhoods, cf.
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