### Proof Complexity of Paris-Harrington Tautologies

**Galesi, N ***(Università degli Studi di Roma La Sapienza)*

Monday 26 March 2012, 11:00-11:30

Seminar Room 1, Newton Institute

#### Abstract

We study the proof complexity of Paris-Harrington’s Large Ramsey Theorem for bicolorings of graphs. We prove a conditional lower bound in Resolution and a upper bound in bounded-depth Frege. The lower bound is conditional on a (very reasonable) hardness assumption for a weak (quasi-polynomial) Pigeonhole principle in Res(2). We show that under such assumption, there is no refutation of the Paris-Harrington formulas of size quasi-polynomial in the number of propositional variables. The proof technique for the lower bound extends the idea of using a combinatorial principle to blow-up a counterexample for another combinatorial principle beyond the threshold of inconsistency. A strong link with the proof complexity of an unbalanced Ramsey principle for triangles is established. This is obtained by adapting some constructions due to Erdo ̋s and Mills.