The Erdős–Pósa theorem for cycles
A Lean 4 / Mathlib formalization of the Erdős–Pósa theorem: for every k, every finite simple graph either contains k vertex-disjoint cycles or has a set of at most c (k + 1)(log₂(k + 1) + 1) vertices meeting every cycle, for a fixed constant c (the proof gives c = 65); together with the qualitative Erdős–Pósa property and the easy converse that a feedback vertex set of size s rules out s + 1 disjoint cycles. Mathlib has acyclic graphs but not this theorem.
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