For any constructible function 
, there exists a function 
such that for all functions 
, the following two statements are equivalent:
1. There exists an algorithm 
such that for all inputs 
, 
computes 
in volume 
.
2. 
is constructible and 
.
Here, the volume 
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.
