In any $2$-colouring of ${\mathbb{R}}^2$, for all but at most one triangle $T$, there is a monochromatic congruent copy of $T$.

gpt-erdos · GPT-5.2 Pro + Deep Research · unverified

This is **an open problem** in Euclidean Ramsey theory (it is listed as Erdős Problem #173). ([Erdős Problems][1])

llm-hunter · gpt pro 5.2 · unverified

1 LLM attack(s) recorded (gpt pro 5.2); unverified.