## Monday, July 24, 2006

### Favorite Theorems: The Permanent

June Edition

The permanent of a matrix has a simple form

Perm(A)=Σσ a1&sigma(1)a2&sigma(2)···an&sigma(n)

where A is an n×n matrix, aij is the element in the ith row and jth column and the sum is taken over all permutations σ of {1,…,n}.

Very similar to the determinant except without the negations and a 0-1 permanent counts the number of perfect matchings on a bipartite graph. But while we can compute the determinant quickly and easily check if a graph has a perfect matching the permanent turns out to have an apparently much higher complexity.

Leslie Valiant, The Complexity of Computing the Permanent, TCS 1979.

Valiant shows that computing the permanent, even for 0-1 matrices, is #P-complete, i.e., as hard as counting the number of solutions for NP problems.

While an important hardness result in its own right, Valiant's theorem becomes even more important with later work. Toda's theorem implies the permanent is hard for the entire polynomial-time hierarchy. The permanent also has two very important properties:

1. The permanent on n×n matrices is easily reducible to solving the permanent on (n-1)×(n-1) matrices.
2. The permanent is a multilinear polynomial on the entries of A.
Those two facts, combined with Valiant's and Toda's theorems, helped the permanent play a critical role in the development of interactive proofs and in some derandomization results, most notably Impagliazzo-Wigderson and Impagliazzo-Kabanets.

1. "Other nice results that are worth mentioning..."

The paper on Clifford algebras has a "result"? Who knew?

2. Another nice result on the permanent,
also due to Valiant, is its completeness
for the class VNP of "easily definable"
families of polynomials.
This implies that the permanent can be
evaluated in a polynomial number of arithmetic operations iff all
"easily definable" families of polynomials
can.

3. Another interesting result on the permanent:

Computing Permanents over Fields of Characteristic 3: Where and Why It Becomes Difficult (extended abstract). FOCS 1996