# determining if a matrix of linear forms represents a non-degenerate matrix

Let $k$ be a field with $p$ elements. Consider the following computational problem

Input: a natural number $n$, $n^2$ linear forms $M_{ij}$, $i,j=1,\ldots n$ in $n^2$ variables $X_{11}, \ldots X_{nn}$.

Problem: Is there an assignement of values to the variables $X_{ij}$ so that the matrix $M_{ij}$ is invertible?

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Question: What is known about algorithms for this problem?

As usually, let's assume the addition and multiplication in the field to have computational cost $1$.

The naive algorithm of checking each assignment of the variables $X_{ij}$ takes time bounded by a polynomial in $p^{n^2}$. I'd be interested to know if there is an improvement to polynomial in $p^n$ (or better).

The special case when each $M_{ij}$ is equal either to $0$ or to $X_{ij}$ is solved by Emil Jeřábek on the mathoverflow thread linked to below.

• I've posted it also to mathoverflow.net/questions/82804/… Dec 6, 2011 at 17:52
• Google for "Polynomial identity testing" Dec 6, 2011 at 20:45
• @MarkusBläser: I wanted to give a similar answer as yours. Yet, I am not sure what can be said on this particular problem, in terms of a deterministic algorithm, using the current results on PIT. Dec 7, 2011 at 7:45
• @Bruno: Derandomizing the problem would imply circuit lower bounds. This problem is considered under the name SDIT (Symbolic Determinant Identity testing) by Kabanets & Impagliazzo. (Comput. Complexity 13, 2004) Dec 7, 2011 at 8:47
• @MarkusBläser: your previous comment should be made an answer. I would add that Gurvits has given a deterministic algorithm for some special cases of SDIT (dx.doi.org/10.1145/780542.780545). He refers to SDIT as "Edmonds' problem". Dec 11, 2011 at 0:25

• Thanks, but this doesn't really answer the question whether you can improve from $p^{n^2}$ to p^n. See the discussion on mathoverflow - there it is shown how to embed instances of 3-SAT of size n into $n\times n$ matrix. But this just shows that one shoudn't hope for an algorithm better than p^n. I made it precise in this question Dec 14, 2011 at 14:47