Number theoryDifficulty 7.0Prove itBMO Round 1 · United Kingdom · 2023
For each integer n≥1, let f(n) be the number of lists of different positive integers starting with 1 and ending with n, in which each term except the last divides its successor. Prove that for each integer N≥1 there is an integer n≥1 such that N divides f(n). (So f(1)=1, f(2)=1 and f(6)=3.)
This one wants a proof. Work it on paper, then check yourself against the publisher's own solution, linked below. Be honest about it: the record is only any use to you if it is.