Daily puzzle · 2026-10-29 · Permutations

Mathbound Daily Puzzle — 2026-10-29 — Permutations Solution

Level 3

How many involutions (bijections

f

with

f\circ f=\mathrm{id}

) exist on a set of

11

elements?

Answer: B

Key idea: $I_n=I_{n-1}+(n-1)I_{n-2}$ — not a $4$ -element listing drill.

Set it up: Compute the recurrence up to $n=11$ .

Solution: $35696$ .

Mathbound Daily Puzzle — 2026-10-29 — Permutations Solution