A function f(x) is said to be constructible if some algorithm F computes it, in binary, within volume O(f(x)), i.e., V_(F(x)) = O(f(x)). Here, the volume V_(A(x)) is the combined number of active edges during all steps, which is the number of state-changes needed to run a certain Turing machine on a particular input.

computable function | Rabin's compression theorem