# A nowhere-zero point in a linear mapping

 Importance: High ✭✭✭
 Author(s): Jaeger, Francois
 Subject: Combinatorics » Matrices
 Keywords: invertible nowhere-zero flow
 Recomm. for undergrads: no
 Prize: none
 Posted by: mdevos on: March 8th, 2007
Conjecture   If is a finite field with at least 4 elements and is an invertible matrix with entries in , then there are column vectors which have no coordinates equal to zero such that .

The motivation for this problem comes from the study of nowhere-zero flows on graphs. If is the directed incidence matrix of a graph , then a nowhere-zero -flow on is precisely a vector so that has all entries nonzero, and . The above conjecture is similar, but is for general (invertible) matrices. Alon and Tarsi have resolved this conjecture for all fields not of prime order using their polynomial technique.

Definition: Say that a matrix is -choosable if for all with and for all with , there exists a vector and a vector so that . Note that every matrix is -choosable, but that an matrix is -choosable if and only if it is invertible.

Alon and Tarsi actually prove a stronger property than Jaeger conjectured for fields not of prime order. They prove that if has characteristic , then every invertible matrix over is -choosable. This result has been extended by DeVos [D] who showed that every such matrix is -choosable. Yang Yu [Y] has verified that the conjecture holds for matrices with entries in when .

Jaeger's conjecture is true in a very strong sense for fields of characteristic 2. DeVos [D] proved that every invertible matrix over such a field is -choosable for every . The following conjecture asserts that invertible matrices over fields of prime order have choosability properties nearly as strong.

Conjecture   [The choosability in conjecture (DeVos)] Every invertible matrix with entries in for a prime is -choosable for every .

This is essentially the strongest choosability conjecture one might hope to be true over fields of prime order. I (M. DeVos) don't have any experimental evidence for this at all, so it could be false already for some small examples. However, I suspect that if The permanent conjecture is true, that this conjecture should also be true. In any case, I (M. DeVos) am offering a bottle of wine for this conjecture.

## Bibliography

[A] N. Alon, Combinatorial Nullstellensatz, Combinatorics Probability and Computing 8 (1999) no. 1-2, 7-29. MathSciNet

[AT] N. Alon, M. Tarsi, A Nowhere-Zero Point in Linear Mappings, Combinatorica 9 (1989), 393-395. MathSciNet

[BBLS] R. Baker, J. Bonin, F. Lazebnik, and E. Shustin, On the number of nowhere-zero points in linear mappings, Combinatorica 14 (2) (1994), 149-157. MathSciNet

[D] M. DeVos, Matrix Choosability, J. Combinatorial Theory, Ser. A 90 (2000), 197-209. MathSciNet

[Y] Y. Yu, The Permanent Rank of a Matrix, J. Combinatorial Theory Ser. A 85 (1999), 237-242. MathSciNet

* indicates original appearance(s) of problem.