July 10, 2026: Ethan Knight announced GPT-5.6 Sol Ultra produced Cycle Double Cover Conjecture proof using 64 subagents in under one hour. OpenAI published 3-page PDF and prompt on CDN.
Proof reduces to loopless cubic graphs, uses nowhere-zero F_2^3 flow, linear algebra compatibility step. Attributed to GPT 5.6 Sol Ultra; writeup by Codex with GPT 5.6 Sol.
Thomas Bloom (Manchester) praised as “very nice” and “elementary” but noted missing 1983 Bermond-Jackson-Jaeger citations. No peer review or Lean/Coq machine check. Wikipedia conjecture page updated same day noting preprint.