MathDB
Combinatorics from IMO 3rd absolute place

Source: Kyiv City MO 2024 Round 2, Problem 10.4

February 4, 2024
combinatoricspermutations

Problem Statement

There are n1n \geq 1 notebooks, numbered from 11 to nn, stacked in a pile. Zahar repeats the following operation: he randomly chooses a notebook whose number kk does not correspond to its location in this stack, counting from top to bottom, and returns it to the kkth position, counting from the top, without changing the location of the other notebooks. If there is no such notebook, he stops.
Is it guaranteed that Zahar will arrange all the notebooks in ascending order of numbers in a finite number of operations?
Proposed by Zahar Naumets