Menger's theorem, Hamiltonian cycles, long cycles and pancyclicity
A Lean 4 / Mathlib formalization of Menger's theorem for finite graphs (set form with a minimum separator and as many vertex-disjoint paths, two-vertex form, edge form for Mathlib's IsEdgeReachable, and the global edge form for Mathlib's IsEdgeConnected) and of classical sufficient conditions for Hamiltonian cycles: the Bondy–Chvátal closure step, Ore's and Dirac's theorems, and the Chvátal–Erdős theorem (k-connected with independence number at most k implies Hamiltonian), whose proof uses Menger's theorem through the fan lemma. Version 2 adds the Erdős–Gallai theorems on long paths and…
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