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Homework Statement
So, if X has a countable basis {Bn}, then every basis C for X contains a countable basis for X.
The Attempt at a Solution
First of all, consider all the intersections of elements of C of the form Ci[tex]\cap[/tex]Cj. For every x in the intersection (if it's non empty), choose a basis element Bx contained in the intersection. There are at most countable such elements. Then, for every Bx, choose an element Cx of C contained in Bx, and containing x. There are at most countable such elements, too. Now do this for all the possible intersections Ci[tex]\cap[/tex]Cj.
Since X has a countable basis, every open covering for X contains a countable subcollection covering X. Apply this to C in order to obtain a countable subcollection C'. We need to show that C' is a basis for X.
First, C' obviously covers X, so there's only one more condition left - we need to show that if x is in the intersection Ci[tex]\cap[/tex]Cj of elements of C', then there exists an element Ck from C' such that x[tex]\in[/tex]Ck[tex]\subseteq[/tex]Ci[tex]\cap[/tex]Cj.
The part I'm not sure about is this: if C' is countable, then the number of intersections of two elements of C' is countable. If this is true, then for each intersection, take the countable number of sets Cx from the first paragraph, and add it to C'. A countable union of countable sets is countable, hence the new collection C'' is a countable subcollection of C which is a basis for X.