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MathNet 2021 — Part 05 Question 46

Math Olympiad Vault · MathNet 2021 — Part 05

Question 46

There are $N$ lockers, labeled from $1$ to $N$, placed in clockwise order around a circular hallway. Initially, all lockers are open. Ansoon starts at the first locker and always moves clockwise. When she is at locker $n$ and there are more than $n$ open lockers, she keeps locker $n$ open and closes the next $n$ open lockers, then repeats the process with the next open locker. If she is at locker $n$ and there are at most $n$ lockers still open, she keeps locker $n$ open and closes all other lockers. She continues this process until only one locker is left open. What is the smallest integer $N \gt 2021$ such that the last open locker is locker $1$?