text: Sacara formal, "X" kaasup aljabar σ mun jeung ukur mun ("jika dan hanya jika", "if and only if") miboga pasipatan di handap ieu: From 1 and 2 it follows that "S" is in "X"; from 2 and 3 it follows that the σ-algebra is also closed under countable intersections (via De Morgan's laws)