Assuming you have decent proof checking software, is it possible that this solution was achieved by throwing GPT at the problem a couple hundred thousand times until it passed the proof checker?
So I’m just asking if the proof checking software is capable of evaluating this proof. Because if it is, that makes the brute force approach a lot more feasible as you reduce human review overhead significantly.
If it is, that would imply you could run the prompt through the LLM as many times as you want until you “strike gold” so to speak.
As far as whether something like Lean could evaluate this proof: sure, if it were mechanized rigorously. But the amount of work that takes to do varies with both subject and complexity of result. In this case, from what other people are saying, the infrastructure for doing graph theory proofs like this isn't as built up as it is for some other areas of mathematics, so it might take a while.
What I'm more saying is that we're a ways away from being able to straightforwardly go from an LLM having a paper proof to having that proof formalized in Lean in the general case. Not so much because it's hard for LLMs, more just because it's hard in general unless all that background work has already been done. As more and more of foundational mathematics gets mechanized, it will be easier and easier to check your work in Lean while you work on the proof. For example, AFAIK unit distance has already been mechanized (though the quality of the mechanization effort sounds not great, it still greatly increases our assurance in the proof's correctness).
Unfortunately in my experience that's not really the case. For me, very often GPT 5.5 (which was a good deal better than Opus at this kind of task) would just get stuck for long periods when working in a logic like Iris. It wouldn't necessarily outright prove nonsense, but it would vastly overclaim what it had proved and failed to get anywhere without a lot of hinting. 5.6 is hopefully a lot better about this.
Lemma 2.2 specifically "feels" new to me. You can get part of the way by duct-taping several papers together (playing along at home: I found Tutte 1954, Bermond–Jackson–Jaeger 1983, Máčajová–Škoviera 2005, Zaslavsky 1982. interestingly, only Tutte appears in the works cited). But it's surprising you'd think to pick those, and surprising it works, because you still need a genuinely novel parity argument at the end. Those steps individually are all pretty simple, knowing to chain that chain together, isn't.
The guess-against the checker paradigm is real (ie AlphaProof), and something like that was probably involved here. But this area of graph theory isn't in mathlib, you need to write the proof checker first, and then you need to know what kind of proof checker you need to write (or just do a brute force search for new proof checkers). Probably how you got this result is have a recursive tree of agents until you divide into small enough subproblems.
At a certain point you need a philosopher to figure out what that "means", ie if you have a big enough tree of small enough subproblems, some of the "magic" so to speak moves out of the proof checkers and into the way the tree got structured.