On computability and tractability for infinite sets.

We propose a definition for computable functions on hereditarily definable sets. Such sets are possibly infinite data structures that can be defined using a fixed underlying logical structure, such as (N, =). We show that, under suitable assumptions on the underlying structure, a programming language called definable while programs captures exactly the computable functions. Next, we introduce a complexity class called fixed-dimension polynomial time, which intuitively speaking describes polynomial computation on hereditarily definable sets. We show that this complexity class contains all functions computed by definable while programs with suitably defined resource bounds. Proving the converse inclusion would prove that Choiceless Polynomial Time with Counting captures polynomial time on finite graphs.