= Solution
On a fixed underlying set, a <pi-system> is a nonempty family closed under finite intersections. A <Dynkin system>, also called a d-system, contains the whole underlying set, is closed under complements relative to it, and under countable disjoint unions. Equivalently it is closed under differences of nested members and increasing countable unions. A <sigma-algebra> contains the whole set, is closed under complements and arbitrary countable unions, hence also countable intersections. A Dynkin system need not be a sigma-algebra unless additional intersection closure is available.
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