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HMMT 2010 C8

  • Combinatorics
  • Answer

How many functions ff from {−1005,…,1005}\{-1005, \ldots, 1005\} to {−2010,…,2010}\{-2010, \ldots, 2010\} are there such that the following two conditions are satisfied?

  • If a<ba<b then f(a)<f(b)f(a)<f(b).
  • There is no nn in {−1005,…,1005}\{-1005, \ldots, 1005\} such that ∣f(n)∣=∣n∣|f(n)|=|n|.

Dataset text NuminaMath olympiads. Quoted with source.

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\binom{4020}{1005}^{2}

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