Classification of Finite Simple Groups Formalized in Lean

A closer look at Classification of Finite Simple Groups Formalized in Lean

What this project is

A complete formalization in Lean of the first-generation proof of the classification of finite simple groups, with every logical step checked by Lean's kernel.

The site presents the Lean formalization, links to the Lean repository, and related material such as an accompanying paper about a discovered issue and a blog post telling the story.

What you can read and follow here

The project page describes the formalization, links to the Lean code (the Lean Repo), and displays a corpus of 139 source works used in the formalization; each block represents a paper or book and is sized by the statements it supplies.

It also highlights a specific result from the work, "The Unbalanced Group Theorem," and links to a paper explaining a found counterexample and an alternative argument built on Solomon's 1976 theory of maximal 2-components.

Who contributed and who might care

Core and other contributors are listed by name, with roles noted such as mathematical contributors, engineering contributors, principal investigator, and founding mathematician.

This will be useful to mathematicians and formal proof engineers interested in the classification of finite simple groups, formal proof development in Lean, or detailed provenance of a large formalization effort.

Questions people ask about Classification of Finite Simple Groups Formalized in Lean

Does the site show the Lean code?

Yes - the page points to the Lean Repo for the Lean code itself.

What is the Unbalanced Group Theorem entry about?

It reports a new group theory result that arose during the formalization, explains a counterexample to a published closure assertion, and gives an alternative argument for the simple-group case.

How large is the source corpus used?

The page indicates a corpus of 139 source works used in the formalization.

Classification of Finite Simple Groups Formalized in Lean was shared on NexURLs by Nexis on November 1, 2026, in Tech & AI. So far it has 1 like from members.