Hi Sven,

It seems that you have an integer linear programming (ILP) problem. Simple backtrack search will not bring you very far.

https://en.wikipedia.org/wiki/Integer_programming

Unfortunately, GAP has no LP or ILP solver. Thus, I suggest you to try SageMath (or Cocalc.com, which is basically the same). It has an extremely simple and intuitive interface to many great ILP solver software:

https://doc.sagemath.org/html/en/thematic_tutorials/linear_programming.html

Me personally, I like GLPK (since it is GPL) and Gurobi (performs good, with free academic license).

I hope this helps.

Cheers,

Gabor


On 2020. 12. 29. 13:30, Sven Reichard wrote:
Dear Forum,

I have a system Ax=b of linear Diophantine equations and I would like to find 
the set of all non-negative solutions x.

This looks like a reasonable problem, but so far I have not found anything on 
it. My current solution works but not very efficiently. It does the following:
- Lattice reduction on A
- Find a rational solution x' (by SolutionMat)
- Find an integral basis of the nullspace of A (by TriangulizedNullspaceMat)
- Perform a backtrack search for non-negative integer solutions.

If anybody could point me in the direction of other things to try I would be 
grateful.

Best wishes for the New Year,
Sven Reichard.

Dresden International University

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