A FrontierMath Major Advance problem got solved, and the problem had no valid counterexample.

Epoch AI logged the first Major Advance tier solution, on emptiness of the core in approval-based committee elections, open since Aziz, Brill and colleagues posed it in 2017. Becker, Greger and Peters worked interactively with GPT-6 Astra, and Peters said he doubts the team would have found the proof without it. Epoch classifies it as human plus AI. Epoch AI The proof shows an empty core is impossible, so the benchmark problem as stated had nothing to find.