quasi
Posts:
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Registered:
7/15/05


Re: Topology & Sigma Algebra
Posted:
Jun 19, 2013 4:28 PM


agapito6314 wrote:
>The Euclidean topology on R (E) is that generated by the >open intervals (x,y), closed under finite intersections and >arbitrary unions. > >The Borel sigma algebra (B) also generated by the open >intervals, is closed under complementation and countable >intersections. > >It appears as if some subsets of R are included in one and >not the other. Is that the case? If so, can someone please >supply examples of a set in E and not in B, and vice versa.
First, note that E doesn't need arbitrary unions  countable unions suffice (every open interval contains a rational number).
Also B gets countable unions via DeMorgan's law, hence B contains all open sets.
Thus, E is a subset of B.
However B contains sets which not open, hence not in E, for example [0,1].
quasi

