PH = PSPACE

TL;DR

This paper proves that PSPACE equals the 4th level of the polynomial hierarchy, using a novel approach involving skolemization, XOR analysis, and local polynomial-time procedures to establish the equivalence.

Contribution

It demonstrates that PSPACE is equal to the 4th level of the polynomial hierarchy, providing a new proof and implications for complexity theory.

Findings

PSPACE equals the 4th level of PH.

A polynomial-time method to analyze XOR dependencies.

Implications for relativization and oracle results.

Abstract

In this paper we show that PSPACE is equal to 4th level in the polynomial hierarchy. We also deduce a lot of important consequences. True quantified Boolean formula is a generalisation of the Boolean Satisfiability Problem, where determining of interpretation that satisfies a given Boolean formula is replaced by existence of Boolean functions that makes a given QBF to be tautology. Such functions are called the Skolem functions. The essential idea of the proof is to show that for any quantified Boolean formula $\phi$ we can obtain a formula $\phi'$ which is in the 4th level of the polynomial hierarchy, no more than polynomial in the size of a given $\phi$, such that the truth of $\phi$ can be determined from the truth of $\phi'$. The idea is to skolemize, and then use additional formulas from the 2nd level of the polynomial hierarchy inside the skolemized prefix to enforce that the skolem variables indeed depend only on the universally quantified variables they are supposed to. However, some dependence is lost when the quantification is reversed. It is called "XOR issue" because the functional dependence can be expressed by means of an XOR formula. Thus, it is needed to locate these XORs, but there is no need to locate all chains with XORs: any chain includes a XOR of only two variables. The last can be done locally in each iteration (keep in mind the algebraic normal form (ANF)), when all arguments are specified, i.e. as a polynomial subroutine. Relativization is defeated due to the well-known fact: PH = PSPACE iff second-order logic over finite structures gains no additional power from the addition of a transitive closure operator. The exchange is possible due to finite possibilities for arguments. So, the theorems with oracles are not applicable since a random oracle is an arbitrary set. And that's why PH is infinite relative to a random oracle with probability 1.