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Research Article on the Baire's Category Theorem As an Important International Journal of Modern Computation, Information and Communication Technology 2019;2(12):71-74. ISSN: 2581-5954 http://ijmcict.gjpublications.com Research Article On the Baire’s Category Theorem as an Important Tool in General Topology and Functional Analysis Amos Otieno Wanjara Kaimosi Friends University College, School of Mathematics, Statistics, and Actuarial Science, P.O BOX 385-50309, Kaimosi-Kenya. *Corresponding author’s e-mail: [email protected] Abstract The problems connected with Topological spaces, Metric spaces, Banach spaces, Boundedness and continuous mappings lie at the heart of Baire’s Category theorem. In this paper, we give a short survey on some classical results due to the Baire’s Category theorem in the field of general Topology and Functional analysis. Keywords: Category; Interior of asset; empty interior; Baire space; open mapping; locally compact Hausdorff space; Metric space. Introduction countably many sets each of The Baire’s category theorem is an important which is rare in X tool in general topology and functional analysis. iii. Nonmeager (or of second The theorem was proved by Rene’-Louis Baire category) in X if M is not meager in his 1899 doctoral thesis [1]. The proof of in X. Baire-category theorem made it possible to Definition 1.1 (Interior of a subset) [6, derive from it the uniform boundedness theorem definition 2.0] as well as the open mapping theorem. The If A is a subset of a space X, the interior of A is uniform boundedness theorem (or uniform the union of all open sets of X that are contained boundedness principle) by S. Banach and H. in A. Steinhaus (1927) [2] is of great importance. Infact throughout analysis there are many Definition 1.2 (Empty interior) [7, definition instances of results related to this theorem, the 1.0] earliest being an investigation by H.Lebesgue To say that A has empty interior is to say that A (1909) [3]. Baire category theorem has various contains no open set of X other than the empty other applications in functional analysis and is set. the main reason why category enters into numerous proofs for instance the more advace Definition 1.3 (Baire space) [8, definition 2.1] books by R.E Edwards (1965) and J.L Kelley A space X is said to be Baire space if the and I. Namiaka (1963) [3,4]. following conditions hold: Given any countable Research Methodology collection { } of closed sets of X, each of which has empty interior in X, their union Definition 1.0 (Category) [5, definition 4.1] also has empty interior in X. A subset M of a metric space X is said to be:) Definition 1.4 (Metric space) [9, definition 4.3] i. Rare (or nowhere dense) in X if its closure has no interior A metric space is a pair (X, d) where X is a set points. and d is a metric on X (or distance function on ii. Meager ( or of the first category) X), that is a function defined on XxX such that in X if M is the union of for every x, y, z, in X we have: i. d is real-valued, finite and non-negative Received: 03.12.2019; Received after Revision: 22.12.2019; Accepted: 23.12.2019; Published: 25.12.2019 ©2019 The Authors. Published by G. J. Publications under the CC BY license. 71 Wanjara, 2019. On the Baire’s category theorem as an important tool in general topology and functional analysis ii. d (x,y)=0 iff x=y Suppose that X is a complete metric space and iii. d (x,y)=d (y, x) (symmetry) that are closed sets, none containing iv. d (x,y) ≤d (x ,z)+d(z, y) (triangle any open Ball, .Then is the whole inequality) of X. Definition1.5 (compactness) [10, definition 1.0] Theorem 1.8.6 There is a continuous function A metric space X is said to be compact if every which is not differentiable at any point in (0,1). sequence in X has a convergent subsequence. A subset M of X is said to be compact if M is Theorem 1.8.7 (Open mapping theorem, compact considered as a subspace of X, that is Bounded inverse theorem) every sequence in M has a convergent A bounded linear operator T from a Banach subsequence whose limit is an element of M. space X onto a Banach space Y is an open Definition1.6 (Local compactness) [11, mapping. Hence if T is bijective, is definition 3.2] continuous and thus bounded. A metric space X is said to be locally compact if Theorem 1.8.8 (Closed graph theorem) every point of X has a compact neighborhood. Let X and Y be Banach spaces and Definition 1.7 (Hausdorff space) [12, definition a closed linear operator, where D(T) . Then if 3] D(T) is closed in X, the operator T is bounded. A topological space X is called a Hausdorff Theorem 1.8.9 (Uniform Bounded theorem) space if for each pair of disjoint points of Let ( ) be a sequence of bounded linear X, there exists neighbourhoods of operators from a Banach space X into respectively that are disjoint. a normed linear space Y such that is Theorem 1.8.0 bounded for every x in X, say If X is a compact Hausdorff space or a complete where is a real metric space, then X is a Baire space. number. Then the sequence of the norms Theorem 1.8.1 (Baire category theorem- is bounded, that is there is a C such that Functional Analysis) [13, theorem 1.1] (n=1,2,…) If a metric space X≠ is complete, it is non- Theorem 1.9.0 (theorem of continuous meager in itself. Hence if X≠ is complete and mapping) then atleast one A mapping T of a metric space X into a metric contains a non-empty open subset. space Y is continuous iff the inverse image of Theorem1.8.2 (BCT1-General topology) any open subset of Y is an open subset of X because T is open. Every complete metric space is a Baire space. More generally, every topological space is Theorem 1.9.1 (Theorem of inverse operator) homeomorphic to an open subset of a complete Let X and Y be vector spaces, both real or both pseidometric space is a Baire space. Thus every convex. Let be a linear operator completely metrixable topological space is a with domain D(T) and range R(T) . Then Baire space. a. the inverse exists if Theorem 1.8.3 (BCT2-General topology) Tx=0 implying x=0 Every locally compact Hausdorff space is a b. If exists,it is a linear operator Baire space c. If dimD(T)=n<∞ and exists, then Theorem 1.8.4 (Baire category) dim R(T)=dim D(T) Suppose that X is a complete metric space and Theorem 1.9.2 (Theorem of continuity and that is a sequence of dense open sets. boundedness) Then is non-empty. Let be a linear operator, where Theorem1.8.5 (Baire category) D(T) and X,Y are normed linear spaces. Then: ©2019 The Authors. Published by G. J. Publications under the CC BY license. 72 Wanjara, 2019. On the Baire’s category theorem as an important tool in general topology and functional analysis a. T is continuous iff T is bounded theorem of inverse operator, theorem 1.9.1, b. If T is continuous at a single point, it is is linear and it is bounded by the theorem of continuous. continuity and boundedness, theorem 1.9.2 Results and discussions Baire category theorem also shows that every Baire category theorem, theorem 1.8.2 is used in complete metric space with no isolated points is functional analysis to prove the following uncountable. (If X is a countable complete theorems: open mapping theorem 1.8.7, the metric space with no isolated points, then each closed graph theorem1.8.8 and the uniform singleton {x} in X is nowhere dense and so X is boundedness theorem 1.8.9. of first category in itself) in particular, this proves that the set of all real numbers is Proof of theorem 1.8.9 (Uniform boundedness uncountable. theorem) Baire’s category theorem shows that each of the For every , let be the set of all following is a Baire space: . is closed. In deed for i. The space of real numbers every x in the closure of there exists a sequence ( ) in covering x. This means that ii. The irrational numbers, with the metric for every fixed n we have and defined by ,where n is the obtains because is continuous first index for which the continued and so is the norm. fraction expansions of x and y differ(T2 Since X is complete, Baire’s theorem implies is a complete metric space) that some contains an open ball, say iii. The cantor set is a Baire space. Let x be arbitrary, In Topology, by Baire’s category theorem, not zero. We set . theorem 1.8.3, every finite-dimensional Then so that .This implies Hausdorff manifold is a Baire space, since it is that we must have Also locally compact and Hausdorff. This is so even since .This implies that for a non- paracompact (hence non metrizable) ).This yields for all n. manifolds such as the long line Conclusions The proof of Baire-category theorem made it Hence n , where possible to derive from it the uniform boundedness theorem as well as the open mapping theorem.
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