Does the polynomial hierarchy collapse if onto functions are invertible?

The class TFNP, defined by Megiddo and Papadimitriou, consists of multivalued functions with values that are polynomially verifiable and guaranteed to exist. Do we have evidence that such functions are hard, for example, if TFNP is computable in polynomial-time does this imply the polynomial-time hierarchy collapses? By computing a multivalued function in deterministic polynomial-time we mean on every input producing one of the possible values of the function on that input. We give a relativized negative answer to this question by exhibiting an oracle under which TFNP functions are easy to compute but the polynomial-time hierarchy is infinite. We also show that relative to this same oracle, P not equal UP and TFNP(NP) functions are not computable in polynomial-time with an NP oracle

Quantum Merlin-Arthur Proof Systems: Are Multiple Merlins More Helpful to Arthur?

Quantum Merlin-Arthur proof systems are a weak form of quantum interactive proof systems, where mighty Merlin as a prover presents a proof in a pure quantum state and Arthur as a verifier performs polynomial-time quantum computation to verify its correctness with high success probability. For a more general treatment, this paper considers quantum “multiple-Merlin”-Arthur proof systems in which Arthur uses multiple quantum proofs unentangled each other for his verification. Although classical multi-proof systems are easily shown to be essentially equivalent to classical single-proof systems, it is unclear whether quantum multi-proof systems collapse to quantum single-proof systems. This paper investigates the possibility that quantum multi-proof systems collapse to quantum single-proof systems, and shows that (i) a necessary and sufficient condition under which the number of quantum proofs is reducible to two and (ii) using multiple quantum proofs does not increase the power of quantum Merlin-Arthur proof systems in the case of perfect soundness. Our proof for the latter result also gives a new characterization of the class NQP, which bridges two existing concepts of “quantum nondeterminism”. It is also shown that (iii) there is a relativized world in which co-NP (actually co-UP) does not have quantum Merlin-Arthur proof systems even with multiple quantum proofs