Human mathematicians are being outcounterexampled

It’s been an interesting few weeks for counterexamples. This post is basically my perspective of what has been going on in the world of formalization, AI tools and, in particular, counterexamples.

Unit distance

Two months ago today (20th May 2026), ChatGPT disproved Erdős’ Unit Distance conjecture in discrete geometry. This is now old news but I had to start somewhere. The announcement was accompanied with testimonies by human mathematicians, many of whom I knew and a few of whom I trusted, saying that they believed the argument (they had been given early access to it and had checked it). The basic structure of the proof is that a profound theorem in number theory due to Golod and Shafarevich from the 1960s could be used to construct a counterexample to the conjecture.

It is now 9 years since I had a mid-life crisis, realised I no longer trusted many human mathematicians when it comes to technical details, discovered Lean, and started to argue that interactive theorem provers should play an important role in the future of mathematics. So of course my first question was “is the counterexample formalized in Lean”. The answer was “no”.

But under a week later (26th May 2026), I got an email from Fields Medallist Mike Freedman. Mike is now the Chief Science Officer for Logical Intelligence, a company cofounded by Turing Award winner and “godfather of AI” Yan LeCun. Mike informed me that their system had autoformalized the entire ChatGPT-generated paper in Lean and could I take a look. I looked, and my post-doc Thomas Browning looked too. And indeed this was what Logical Intelligence had done: they had formalized precisely the statement that the profound theorem of number theory implied the Erdős counterexample. Breakthrough LLM-generated mathematics being formalized in real time. Interesting data point.

Of course there is an elephant in the room here though, the profound theorem of number theory which takes 100+ pages to prove (it needs huge chunks of global class field theory, a theory developed at the beginning of the 20th century and for which there are still no short proofs; it is proving difficult to compress). In 2025 I had run a Clay Summer School with Richard Hill on the formalization of class field theory, and one year later we have nearly done the local case (it is the current PhD project of my student Edison Xie); the global case remained open, and indeed in 2025 formalizing global class field theory seemed like a fantasy.

One month later, on June 26th 2026, my perception of what was possible again changed. Boris Alexeev announced on the Lean Zulip that he had steered ChatGPT to a complete formalization of the Erdős counterexample, assuming nothing beyond the axioms of mathematics. Boris works at OpenAI and had used their new model Sol to do the autoformalization. Boris made the code public and it did not take long for me to realise that somewhere within all this AI-generated (and sometimes horrible, although sometimes decent) code was indeed a proof of some really hard theorems in global class field theory. Also of interest to me was that Sol had generated 1.2 million lines of Lean code in the three weeks that it had worked on the project. Lean’s fantastic (declaration of conflict of interest: I am a maintainer) mathematics library mathlib is only 2.3 million lines of code, and took nine years to write. Perhaps it was at this point that the penny really dropped for me — large AI-generated developments of mathematics are inevitable. One cannot trust AI-generated code so I ran it in a sandbox on my machine (malicious Lean code can run arbitrary commands on your computer — Lean is a programming language, after all). Indeed, it was proving nontrivial theorems about the cohomology of number fields. Wow.

Group schemes of order n

A week after Boris’ revelation, in early July, I was thinking hard about how to run my Formalizing Fermat workshop. This workshop was sponsored by Logos Research, who, like Logical Intelligence (and Harmonic and Axiom AI and Moonshot AI and…) have a tool which can autoformalize mathematics — translating it from human language into Lean — building on mathlib. Logos told me that they were only going to allow 5 people at a time to use their system during the workshop, and there were 25 attendees, so I told all attendees that I would buy them a Claude Max subscription for a month, so they had something to experiment with when it wasn’t their turn for Logos’ tool. The workshop was 6th to 10th July, and the Claude Max subscription would give attendees access to Claude Fable, at least until Tuesday 7th, when it was being switched off. When OpenAI got wind of what I was doing, they also offered all attendees free ChatGPT Pro access for a month; this was a big deal because ChatGPT Sol was coming out on the 9th. So basically all attendees would have access to Sol and Fable for 4 out of the 5 days of the workshop, and Logos’ tool for the entire week. In fact Fable access was not removed on the 7th so we were in even better shape.

I was not sure how good Logos’ tool was going to be, but I wanted a development of the theory of finite flat group schemes in Lean for my ongoing proof of Fermat’s Last Theorem, so I put uploaded some classic papers in the area to Fable and ChatGPT, and got them together to write down an exposition of the theory in natural language. I passed this pdf document over to Logos the day before the workshop, and on the first day of the workshop they said that one of the claims in the pdf was false and they had found an explicit counterexample. Another counterexample! I took a look and indeed the LLM-generated pdf was simply wrong at some point when describing a standard construction; false alarm. I had missed this myself though when reading through the pdf. Interesting how AI had again found a counterexample. I fixed the pdf. I thought it was interesting that the AI didn’t just say “I don’t quite follow this argument”, it instead said “here is a proof that this argument is simply wrong”, a much more powerful statement.

With the development of the theory of finite flat group schemes back on track, I could relax back into the FLT workshop. On Tuesday 7th July I sat opposite Akhil Mathew at lunch; Akhil is a professor of mathematics at UChicago and he was an attendee who had been experimenting with the tools available. We talked about potential questions which AI could work on, and Akhil raised the old question of Grothendieck about whether every finite free group scheme of order n was killed by n. Deligne had proved the result in the commutative case, and Grothendieck had proved it when the base was reduced; Rene Schoof had proved it in more cases, and there had even been a paper by Emiliano Torti published last year, proving it in even more generality. I said that I thought that this was a fabulous thing to get AI thinking about.

The day after the workshop finished, on Saturday 11th July, I got a DM from Akhil telling me that Sol had found a counterexample. He sent me a 12 page pdf. I immediately replied saying that I was not reading AI-generated informal mathematics and could he please formalize the entire thing in Lean. Four hours later he replied again, saying that Fable had autoformalized the entire thing. I scanned over the 1076-line Lean file, checking that the code did not delete all the files on my hard drive (Lean is a programming language, so it can do this). Convinced that it was only theorems, I then compiled it on my laptop and it took me under 5 minutes in total to check that (a) the statement of the claimed theorem used only concepts in mathlib (and thus things like HopfAlgebra can be trusted to mean what mathematicians think of as Hopf algebras) (b) the statement of the claimed theorem was that there was a counterexample and (c) the proof compiled. At this point I knew that we had a counterexample — a group scheme of order 4 which was not killed by 4. I suggested to Akhil that h