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1)prove that an algebra of sets is closed under finite union and intersections. 2)prove that for a semiring S,countable unions and finite intersections of sigma sets are sigma sets.

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What have you tried? Where are you stuck? – jericson Apr 23 '11 at 21:34
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Please don't post your messages in the imperative, as if you were giving an assignment or issuing orders. Give some context (is this self-study? is this an assignment? What course is it for?) and what you've tried so far (as well as where you are stuck). – Arturo Magidin Apr 23 '11 at 21:34
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Hint: induction. – Yuval Filmus Apr 23 '11 at 21:35

closed as not a real question by t.b., Asaf Karagila, Jonas Meyer, Qiaochu Yuan Apr 23 '11 at 22:35

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