In the computational complexity theory of counting problems, a polynomial-time counting reduction is a type of reduction (a transformation from one problem to another) used to define the notion of completeness for the complexity class ♯P.
[1] These reductions may also be called polynomial many-one counting reductions or weakly parsimonious reductions; they are analogous to many-one reductions for decision problems and they generalize the parsimonious reductions.
[2] A polynomial-time counting reduction is usually used to transform instances of a known-hard problem
, both of which must be computable in polynomial time.
[1][2] These two functions must preserve the correctness of the output.
That is, suppose that one transforms an input
It must be the case that the transformed output
is the correct output for the original input
are expressed as functions, then their function composition must obey the identity
Alternatively, expressed in terms of algorithms, one possible algorithm for solving
to transform the problem into an instance of
, solve that instance, and then apply
[1][2] As a special case, a parsimonious reduction is a polynomial-time transformation
on the inputs to problems that preserves the exact values of the outputs.
Such a reduction can be viewed as a polynomial-time counting reduction, by using the identity function as the function
[1][2] A functional problem (specified by its inputs and desired outputs) belongs to the complexity class ♯P if there exists a non-deterministic Turing machine that runs in polynomial time, for which the output to the problem is the number of accepting paths of the Turing machine.
Intuitively, such problems count the number of solutions to problems in the complexity class NP.
is said to be ♯P-hard if there exists a polynomial-time counting reduction from every problem
[1][2] (Sometimes, as in Valiant's original paper proving the completeness of the permanent of 0–1 matrices, a weaker notion of reduction, Turing reduction, is instead used for defining ♯P-completeness.
[3]) The usual method of proving a problem
in ♯P to be ♯P-complete is to start with a single known ♯P-complete problem
and find a polynomial-time counting reduction from
If this reduction exists, then there exists a reduction from any other problem in ♯P to
, obtained by composing a reduction from the other problem to