10.04.2013 Views

1. The Baire category theorem - Aarhus Universitet

1. The Baire category theorem - Aarhus Universitet

1. The Baire category theorem - Aarhus Universitet

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

Definition of first and second <strong>category</strong><br />

First <strong>category</strong><br />

A set A ⊆ S is of first <strong>category</strong> if it is the union of a countable<br />

collection of nowhere dense sets, i.e. if A = ∞ i=1 Ai, where Ai have<br />

empty interior.<br />

Example: <strong>The</strong> rational numbers Q form a set of first <strong>category</strong> in<br />

the real line R. Note that Q itself is not nowhere dense; but it is<br />

the union of a countable collection of sets that are!<br />

Klaus Thomsen <strong>1.</strong> <strong>The</strong> <strong>Baire</strong> <strong>category</strong> <strong>theorem</strong>

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!