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Why is a Dynkin system not a Pi-system

  1. May 25, 2012 #1
    This appears to be the only difference between a sigma-algebra and a Dynkin system:
    • Sigma-algebra is closed under countable union
    • Dynkin system is closed under countable union of disjoint sets

    This seems to result in the D-system not being a pi-system (while the sigma-alg is). Why?

    Many thanks.
  2. jcsd
  3. May 28, 2012 #2
    Hint: think about complements.
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