Baire category theorem




The Baire category theorem (BCT) is an important tool in general topology and functional analysis. The theorem has two forms, each of which gives sufficient conditions for a topological space to be a Baire space.


The theorem was proved by René-Louis Baire in his 1899 doctoral thesis.




Contents






  • 1 Statement of the theorem


  • 2 Relation to the axiom of choice


  • 3 Uses of the theorem


  • 4 Proof


  • 5 See also


  • 6 Notes


  • 7 References


  • 8 External links





Statement of the theorem


A Baire space is a topological space with the following property: for each countable collection of open dense sets {Un}n=1∞{displaystyle {U_{n}}_{n=1}^{infty }}{displaystyle {U_{n}}_{n=1}^{infty }}, their intersection ⋂n=1∞Un{displaystyle textstyle bigcap _{n=1}^{infty }U_{n}}{displaystyle textstyle bigcap _{n=1}^{infty }U_{n}} is dense.



  • (BCT1) Every complete metric space is a Baire space. More generally, every topological space which is homeomorphic to an open subset of a complete pseudometric space is a Baire space. Thus every completely metrizable topological space is a Baire space.

  • (BCT2) Every locally compact Hausdorff space is a Baire space. The proof is similar to the preceding statement; the finite intersection property takes the role played by completeness.


Note that neither of these statements implies the other, since there are complete metric spaces which are not locally compact (the irrational numbers with the metric defined below; also, any Banach space of infinite dimension), and there are locally compact Hausdorff spaces which are not metrizable (for instance, any uncountable product of non-trivial compact Hausdorff spaces is such; also, several function spaces used in Functional Analysis; the uncountable Fort space). See Steen and Seebach in the references below.


  • (BCT3) A non-empty complete metric space, or any of its subsets with nonempty interior, is not the countable union of nowhere-dense sets.

This formulation is equivalent to BCT1 and is sometimes more useful in applications. Also: if a non-empty complete metric space is the countable union of closed sets, then one of these closed sets has non-empty interior.



Relation to the axiom of choice


The proofs of BCT1 and BCT2 for arbitrary complete metric spaces require some form of the axiom of choice; and in fact BCT1 is equivalent over ZF to a weak form of the axiom of choice called the axiom of dependent choices.[1]


A restricted form of the Baire category theorem, in which the complete metric space is also assumed to be separable, is provable in ZF with no additional choice principles.[2] This restricted form applies in particular to the real line, the Baire space ωω, the Cantor space 2ω, and a separable Hilbert space such as L2(R n).



Uses of the theorem


BCT1 is used in functional analysis to prove the open mapping theorem, the closed graph theorem and the uniform boundedness principle.


BCT1 also shows that every complete metric space with no isolated points is uncountable. (If X is a countable complete metric space with no isolated points, then each singleton {x} in X is nowhere dense, and so X is of first category in itself.) In particular, this proves that the set of all real numbers is uncountable.


BCT1 shows that each of the following is a Baire space:



  • The space R{displaystyle mathbb {R} }mathbb {R} of real numbers

  • The irrational numbers, with the metric defined by d(x,y)=1n+1{displaystyle d(x,y)={tfrac {1}{n+1}}}{displaystyle d(x,y)={tfrac {1}{n+1}}}, where n{displaystyle n}n is the first index for which the continued fraction expansions of x{displaystyle x}x and y{displaystyle y}y differ (this is a complete metric space)

  • The Cantor set


By BCT2, every finite-dimensional Hausdorff manifold is a Baire space, since it is locally compact and Hausdorff. This is so even for non-paracompact (hence nonmetrizable) manifolds such as the long line.



Proof


The following is a standard proof that a complete pseudometric space X{displaystyle scriptstyle X}scriptstyle X is a Baire space.


Let Un{displaystyle U_{n}}{displaystyle U_{n}} be a countable collection of open dense subsets. We want to show that the intersection ⋂Un{displaystyle bigcap U_{n}}{displaystyle bigcap U_{n}} is dense. A subset is dense if and only if every nonempty open subset intersects it. Thus, to show that the intersection is dense, it is sufficient to show that any nonempty open set W{displaystyle W}W in X{displaystyle X} X has a point x{displaystyle x}x in common with all of the Un{displaystyle U_{n}}{displaystyle U_{n}}. Since U1{displaystyle U_{1}}{displaystyle U_{1}} is dense, W{displaystyle W}W intersects U1{displaystyle U_{1}}{displaystyle U_{1}}; thus, there is a point x1{displaystyle x_{1}}x_{1} and 0<r1<1{displaystyle 0;<;r_{1};<;1}{displaystyle 0;<;r_{1};<;1} such that:


B¯(x1,r1)⊂W∩U1{displaystyle {overline {B}}(x_{1},r_{1})subset Wcap U_{1}}overline{B}(x_1, r_1) subset W cap U_1

where B(x,r){displaystyle B(x,r)}{displaystyle B(x,r)} and B¯(x,r){displaystyle {overline {B}}(x,r)}{displaystyle {overline {B}}(x,r)} denote an open and closed ball, respectively, centered at x{displaystyle x}x with radius r{displaystyle r} r. Since each Un{displaystyle U_{n}}{displaystyle U_{n}} is dense, we can continue recursively to find a pair of sequences xn{displaystyle x_{n}}x_{n} and 0<rn<1n{displaystyle 0;<;r_{n};<;{frac {1}{n}}}{displaystyle 0;<;r_{n};<;{frac {1}{n}}} such that:


B¯(xn,rn)⊂B(xn−1,rn−1)∩Un{displaystyle {overline {B}}(x_{n},r_{n})subset B(x_{n-1},r_{n-1})cap U_{n}}overline{B}(x_n, r_n) subset B(x_{n - 1}, r_{n - 1}) cap U_n

(This step relies on the axiom of choice.) Since xn∈B(xm,rm){displaystyle x_{n};in ;B(x_{m},r_{m})}{displaystyle x_{n};in ;B(x_{m},r_{m})} when n>m{displaystyle n;>;m}{displaystyle n;>;m}, we have that xn{displaystyle x_{n}}x_{n} is Cauchy, and hence xn{displaystyle x_{n}}x_{n} converges to some limit x{displaystyle x}x by completeness. For any n{displaystyle n}n, by closedness,


x∈B¯(xn,rn).{displaystyle xin {overline {B}}(x_{n},r_{n}).}x in overline{B}(x_n, r_n).

Therefore x∈W{displaystyle x;in ;W}{displaystyle x;in ;W} and x∈Un{displaystyle x;in ;U_{n}}{displaystyle x;in ;U_{n}} for all n{displaystyle n}n.


See also this blog post [1] by M. Baker for the proof of the theorem using Choquet's game.



See also


  • Property of Baire


Notes





  1. ^ Blair 1977


  2. ^ Levy 1979, p. 212




References



  • R. Baire. Sur les fonctions de variables réelles. Ann. di Mat., 3:1–123, 1899.

  • Blair, Charles E. (1977), "The Baire category theorem implies the principle of dependent choices.", Bull. Acad. Polon. Sci. Sér. Sci. Math. Astronom. Phys., v. 25 n. 10, pp. 933–934.


  • Levy, Azriel (1979), Basic Set Theory. Reprinted by Dover, 2002. .mw-parser-output cite.citation{font-style:inherit}.mw-parser-output q{quotes:"""""""'""'"}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/6/65/Lock-green.svg/9px-Lock-green.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-limited a,.mw-parser-output .cs1-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Lock-gray-alt-2.svg/9px-Lock-gray-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Lock-red-alt-2.svg/9px-Lock-red-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{font-size:100%}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}
    ISBN 0-486-42079-5


  • Schechter, Eric, Handbook of Analysis and its Foundations, Academic Press,
    ISBN 0-12-622760-8


  • Lynn Arthur Steen and J. Arthur Seebach, Jr., Counterexamples in Topology, Springer-Verlag, New York, 1978. Reprinted by Dover Publications, New York, 1995.
    ISBN 0-486-68735-X (Dover edition).


  • Theodore W. Gamelin and Robert Everist Greene, Introduction to Topology 2nd edition Dover



External links



  • T. Tao, 245B, Notes 9: The Baire category theorem and its Banach space consequences

  • Encyclopaedia of Mathematics article on Baire theorem




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