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A Sigma-ring or [math]\sigma[/math]-ring, is closely related to rings of sets, and just like algebras of sets are related to rings of sets, so is [math]\sigma[/math]-ring to [math]\sigma[/math]-algebra


A non-empty class of sets [ilmath]S[/ilmath] is a [ilmath]\sigma[/ilmath]-ring if[1]:

  • for [ilmath]A\in S[/ilmath] and [ilmath]B\in S[/ilmath] then [ilmath]A-B\in S[/ilmath]
  • If [math]\{A_n\}_{n=1}^\infty\subset S[/math] then [math]\cup^\infty_{n=1}A_n\in S[/math]

That is to say it is closed under subtraction and countable union


  1. Measure Theory, p24 - Halmos - Graduate Texts in Mathematics (18) - Springer