olymmath

Putnam 1990 A6

  • Session A

If XX is a finite set, let XX denote the number of elements in XX. Call an ordered pair (S,T)(S, T) of subsets of {1,2,…,n}\{1, 2, \dots, n\} admissible if s>∣T∣s > |T| for each s∈Ss \in S, and t>∣S∣t > |S| for each t∈Tt \in T. How many admissible ordered pairs of subsets of {1,2,…,10}\{1, 2, \dots, 10\} are there? Prove your answer.

Curated text Kedlaya Putnam archive. Quoted with source.

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