olymmath

Putnam 2013 B5

  • Session B

Let X={1,2,…,n}X = \{1, 2, \dots, n\}, and let k∈Xk \in X. Show that there are exactly k⋅nn−1k \cdot n^{n-1} functions f:X→Xf: X \to X such that for every x∈Xx \in X there is a j≥0j \geq 0 such that f(j)(x)≤kf^{(j)}(x) \leq k. [Here f(j)f^{(j)} denotes the jjth iterate of ff, so that f(0)(x)=xf^{(0)}(x) = x and f(j+1)(x)=f(f(j)(x))f^{(j+1)}(x) = f(f^{(j)}(x)).]

Curated text Kedlaya Putnam archive. Quoted with source.

Solutions

Human solution Solution 1 by Kiran Kedlaya and Lenny Ng

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