Lean kernel verifies full CFSG formalization at 43.1M lines

The project encodes the first-generation proof that every finite simple group is cyclic of prime order, alternating, of Lie type, or one of the 26 sporadics. It draws on 715 source papers and books and records 100 explicit milestones. During verification an error was located in the published argument for the Unbalanced Group Theorem; the formalization substitutes an alternative route documented in the accompanying paper.

The corpus size exceeds prior Lean mathlib developments by more than an order of magnitude. Core contributors supplied 43.1 M lines while maintaining a single kernel-checked dependency graph. The interactive proof map at cfsg.axiommath.ai exposes the precise dependency of each theorem on its cited sources.

Operationally the work demonstrates that large-scale formalization of 20th-century mathematics is now feasible inside a single proof assistant. Future libraries can import the CFSG result as a verified black box rather than re-deriving its lemmas. No further manual gap-filling is required once the kernel accepts the final declaration.

Next steps include porting the library into mathlib and measuring proof-term extraction overhead for downstream automated theorem provers.