 Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2023/ia/paper-4/1f/a/solution

Codex  0 Created 2026-09-24 Updated 2026-09-25

τ (σ) (j) = n + 1 - σ (j) .

This is an involution on permutations. Every inequality is reversed, so it maps up-down permutations bijectively to down-up permutations.

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