Baire category theorem

The Baire category theorem 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.^[1]

Statement of the theorem

A Baire space is a topological space with the following property: for each countable collection of open dense sets U_n, their intersection ∩ 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 is a complete metric space which is not locally compact (the irrational numbers with the metric defined below), and there is a locally compact Hausdorff space which is not metrizable (uncountable Fort space). See Steen and Seebach in the references below.

• (BCT3) A non-empty complete metric space is NOT the countable union of nowhere-dense sets (i.e, sets whose closure has dense complement).

This formulation is a consequence of 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 require some form of the axiom of choice; and in fact BCT1 is (over ZF) equivalent to a weaker version of the axiom of choice called the axiom of dependent choices. [1]

Uses of the theorem

BCT1 is used 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 of real numbers
• The irrational numbers, with the metric defined by d(x, y) = 1 / (n + 1), where n is the first index for which the continued fraction expansions of x and y differ (this is a complete metric space)
• The Cantor set

By BCT2, every 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 metric space X is a Baire space.

Let U_n be a countable collection of open dense subsets. We want to show that the intersection is dense. For that, let be an open subset. By denseness, there is x₁ and r₁ > 0 such that:

.

(B(x,r) and denote an open ball centered at x with radius r and its closure, respectively.) Recursively, we find x_n and r_n > 0 such that:

as well as r_n < n ^{− 1}.

Since when n > m, we have that x_n is Cauchy, and x_n converges to some limit x. For any n, by closedness,

.

Hence, and for all n.

Notes

References

• 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).