Computational Complexity

Monday, July 24, 2006

 
Favorite Theorems: The Permanent

Posted by Lance

June Edition

The permanent of a matrix has a simple form

Perm(A)=Σ_σ a_1&sigma(1)a_2&sigma(2)···a_n&sigma(n)

where A is an n×n matrix, a_ij 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.

8:15 AM #

1.  Anonymous says:  
   
   Other nice results that are worth mentioning are "Clifford algebras and approximating the permanent" and "A polynomial-time approximation algorithm for the permanent of a matrix with non-negative entries"
   

   10:36 AM, July 24, 2006

2.  Anonymous says:  
   
   "Other nice results that are worth mentioning..."
   
   The paper on Clifford algebras has a "result"? Who knew?
   

   6:40 PM, July 24, 2006

3.  Pascal Koiran says:  
   
   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.
   

   2:44 AM, July 26, 2006