On SAT Solvers and Ramsey-type Numbers

Abstract

We created and parallelized two SAT solvers to find new bounds on some Ramsey-type numbers. For $c > 0$, let $R_c(L)$ be the least $n$ such that for all $c$-colorings of the $[n]\times [n]$ lattice grid there will exist a monochromatic right isosceles triangle forming an $L$. Using a known proof that $R_c(L)$ exists we obtained $R_3(L) \leq 2593$. We formulate the $R_c(L)$ problem as finding a satisfying assignment of a boolean formula. Our parallelized probabilistic SAT solver run on eight cores found a 3-coloring of $20\times 20$ with no monochromatic $L$, giving the new lower bound $R_3(L) \geq 21$. We also searched for new computational bounds on two polynomial van der Waerden numbers, the "van der Square" number $R_c(VS)$ and the "van der Cube" number $R_c(VC)$. $R_c(VS)$ is the least positive integer $n$ such that for some $c > 0$, for all $c$-colorings of $[n]$ there exist two integers of the same color that are a square apart. $R_c(VC)$ is defined analogously with cubes. For $c \leq 3$, $R_c(VS)$ was previously known. Our parallelized deterministic SAT solver found $R_4(VS)$ = 58. Our parallelized probabilistic SAT solver found $R_5(VS) > 180$, $R_6(VS) > 333$, and $R_3(VC) > 521$. All of these results are new.