Preprint:

Arnold Beckmann, Sam Buss, Sy David Friedman, Moritz Müller, and Neil Thapen
Cobham Recursive Set Functions
Annals of Pure and Applied Logic 167, 3 (2016) 335-369.

Abstract: This paper introduces the Cobham Recursive Set Functions (CRSF) as a version of polynomial time computable functions on general sets, based on a limited (bounded) form of $\in$-recursion. This is inspired by Cobham's classic definition of polynomial time functions based on limited recursion on notation. We introduce a new set composition function, and a new smash function for sets which allows polynomial increases in the ranks and the cardinalities of transitive closures. We bootstrap CRSF, prove closure under (unbounded) replacement, and prove that any CRSF function is embeddable into a smash term. When restricted to natural encodings of binary strings as hereditarily finite sets, the CRSF functions define precisely the polynomial time computable functions on binary strings. Prior work of Beckmann, Buss and Friedman and of Arai introduced set functions based on safe-normal recursion in the sense of Bellantoni-Cook. We prove an equivalence between our class CRSF and a variant of Arai's predicatively computable set functions.

Related talk:

"Polynomial Time Computability for Set Functions"
Steklov Institute, Moscow
July 22, 2015.

"Cobham recursion and polynomial time on sets"
Workshop on Sets and Computations
IMS, National University of Singapore
April 13, 2015.

Earlier talks:
"Cobham recursion and polynomial time on sets"
Oberwolfach workshop
November 18, 2014.

"Cobham recursion and polynomial time computability for sets"
Logic seminar, Stanford University
October 24, 2014.

"Cobham Recursive Set Functions"
Infinity Workshop, Vienna Summer of Logic
July 2014.