NullspaceMat( mat )
NullspaceMat returns a base for the nullspace of the matrix mat. The
nullspace is the set of vectors vec such that vec * mat is the
zero vector. The returned base is the standard base for the nullspace
(see BaseMat).
gap> NullspaceMat( [ [ 2, -4, 1 ], [ 0, 0, -4 ], [ 1, -2, -1 ] ] );
[ [ 1, 3/4, -2 ] ]
Note that for large rational matrices NullspaceMat may take very long,
because the entries may become very large during the Gaussian
elimination, even if the final result only contains small integers.
GAP 3.4.4