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Disjoint subsets of an n-element set

Source: Czech and Slovak Olympiad 1972, National Round, Problem 5

July 10, 2024
combinatoricsSubsets

Problem Statement

Determine how many unordered pairs {A,B}\{A,B\} is there such that A,B⊆{1,…,n}A,B\subseteq\{1,\ldots,n\} and A∩B=∅.A\cap B=\emptyset.
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