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  "body": "\n## Summary\n\nA Dynkin system, or λ-system, on a set $X$ is a family of subsets that contains $X$.\nIt is closed under complements and under countable unions of disjoint members.\nIt is weaker than a σ-algebra, because it need not be closed under intersections.\nDynkin systems appear in the π-λ theorem, the standard tool to prove that two measures are equal.\n\n## Explanations\n\n### L1 human\n\nA Dynkin system is the kind of family one gets when one collects all sets on which two probability measures agree.\nSuch a family is stable under complements and disjoint countable unions, but in general not under intersections.\nA small example: on $\\set{1, 2, 3, 4}$, the subsets with an even number of elements form a Dynkin system.\nIt is not a σ-algebra, since $\\set{1, 2} \\cap \\set{2, 3} = \\set{2}$ has one element.\n",
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