Published online by Cambridge University Press: 12 March 2014
We show, under the assumption that factoring is hard, that a model of PV exists in which the polynomial hierarchy does not collapse to the linear hierarchy; that a model of exists in which NP is not in the second level of the linear hierarchy; and that a model of exists in which the polynomial hierarchy collapses to the linear hierarchy.
Our methods are model-theoretic. We use the assumption about factoring to get a model in which the weak pigeonhole principle fails in a certain way, and then work with this failure to obtain our results.
As a corollary of one of the proofs, we also show that in the failure of WPHP (for definable relations) implies that the strict version of PH does not collapse to a finite level.