puzzle(archive/oxford_invariants): Oxford Invariants Puzzle Challenges, Summer 2021, Week 3, Problem 1 (#8656)
puzzle(archive/oxford_invariants): Oxford Invariants Puzzle Challenges, Summer 2021, Week 3, Problem 1 (#8656)

This is a formalisation of a problem posed by the Oxford Invariants.

Co-authored by @b-mehta



Co-authored-by: Anne Baanen <839f20e486a83a313cf46ca4094e2de98129fc5d@users.noreply.github.com>

leanprover-community/mathlib,leanprover-community/mathlib