Sets That Avoid Their Own Size

Problem

Nonempty sets AA and BB satisfy AB={1,2,3,,10}A \cup B = \{1, 2, 3, \dots, 10\} and AB=A \cap B = \varnothing. The number of elements of AA is not an element of AA, and the number of elements of BB is not an element of BB. Find the number of possible sets AA.

Answer

Solution

Difficulty5/10
Topicscombinatorics, Counting, Set Theory, Casework

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