Thursday, 22 August 2013

Is it $\sigma$-ring?

Is it $\sigma$-ring?

Is it true that if a (not empty) class of sets is closed under the
symmetric differences ($A\Delta B:=(A-B)\cup(B-A)$) and countable
intersections, then it is a $\sigma$-ring? I proved that ring. I have
problem with the countable infinite union.

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