olymmath

Putnam 2024 B4

  • Session B

Let nn be a positive integer. Set an,0=1a_{n,0} = 1. For k≥0k \geq 0, choose an integer mn,km_{n,k} uniformly at random from the set {1,…,n}\{1,\dots,n\}, and let an,k+1={an,k+1,if mn,k>an,k;an,k,if mn,k=an,k;an,k−1,if mn,k<an,k.a_{n,k+1} = \begin{cases} a_{n,k} + 1, & \text{if $m_{n,k} > a_{n,k};$} \\ a_{n,k}, & \text{if $m_{n,k} = a_{n,k}$;} \\ a_{n,k}-1, & \text{if $m_{n,k} < a_{n,k}$.} \end{cases} Let E(n)E(n) be the expected value of an,na_{n,n}. Determine lim⁡n→∞E(n)/n\lim_{n\to \infty} E(n)/n.

Curated text Kedlaya Putnam archive. Quoted with source.

Solutions

Human solution Solution 1 by Manjul Bhargava, Kiran Kedlaya and Lenny Ng

The terms of the source let olymmath link to this solution but not copy it. Read the solution (Kedlaya Putnam archive).

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