Editing Probability axioms (section)

=== {{Anchor|Sigma additivity|Finite additivity|Countable additivity|Finitely additive}}Third axiom ===
This is the assumption of [[σ-additivity]]:
: Any [[countable]] sequence of [[disjoint sets]] (synonymous with ''[[Mutual exclusivity|mutually exclusive]]'' events) <math>E_1, E_2, \ldots</math> satisfies
::<math>P\left(\bigcup_{i = 1}^\infty E_i\right) = \sum_{i=1}^\infty P(E_i).</math>
Some authors consider merely [[finitely additive]] probability spaces, in which case one just needs an [[field of sets|algebra of sets]], rather than a [[σ-algebra]].<ref>{{Cite web|url=https://plato.stanford.edu/entries/probability-interpret/#KolProCal|title=Interpretations of Probability|last=Hájek|first=Alan|date=August 28, 2019|website=Stanford Encyclopedia of Philosophy|access-date=November 17, 2019}}</ref> [[Quasiprobability distribution]]s in general relax the third axiom.