For a countable set of disjoint events , , ...,
Countable Additivity Probability Axiom
See also
Countable Additivity, Countable SetExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Countable Additivity Probability Axiom." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/CountableAdditivityProbabilityAxiom.html