Tuesday, December 30, 2014

Product of a Compact and Paracompact Space (6.41.2a)

James Munkres Topology, chapter 6.41, exercise 2a:

MathJax TeX Test Page Show that the product of a paracompact space X and a compact space Y is paracompact.

Proof: Let {Uα} be an open cover of X×Y. For each xX, the space {x}×YY is compact, so let it be covered by finitely many Ux1,...,Uxn with nontrivial intersection with {x}×Y, and let Wx=π1(Uxi). Then {Wx}xX is an open cover of X, so let A be a locally finite open refinement covering X. For each AA, finitely many Ux1,...,Uxn cover A×YWx×Y (for some x), so let CA={(A×Y)Uxi}. We claim C=AACA is a locally finite open refinement of {Uα} covering X×Y.

Every element of each CA is contained in some Uα, so C is clearly a refinement. As well, given z=x×yX×Y, let xAA so that zA×Y=CA implying z is contained in an element of CA and now C covers X. Finally, choose a neighborhood U of x intersecting only finitely many members AiA. Then U×Y is a neighborhood of z that can intersect only among the members of CAi, all of which are finite. Therefore C is locally finite and X×Y is paracompact. 

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