Safe Recursion Over an Arbitrary Structure: PAR, PH and DPH

Considering the Blum, Shub, and Smale computational model for real numbers, extended by Poizat to general structures, classical complexity can be considered as the restriction to finite structures of a more general notion of computability and complexity working over arbitrary structures. In a previous paper, we showed that the machine-independent characterization of Bellantoni and Cook of sequential polynomial time for classical complexity is actually the restriction to finite structures of a characterization of sequential polynomial time over arbitrary structures. In this paper, we prove that the same phenomenon happens for several other complexity classes: over arbitrary structures, parallel polynomial time corresponds to safe recursion with substitutions, and the polynomial hierarchy corresponds to safe recursion with predicative minimization. Our results yield machine-independent characterizations of several complexity classes subsuming previous ones when restricted to finite structures. 1 Partially supported by City University of Hong Kong SRG grant 7001290. Preprint submitted to Elsevier Preprint 1 April 2005