Function: matimagemod
Section: linear_algebra
C-Name: matimagemod
Prototype: GGD&
Help: matimagemod(x,d,&U): basis of the image of the matrix x modulo d.
Doc: gives a Howell basis (unique representation for submodules of~$(\Z/d\Z)^n$)
 for the image of the matrix $x$ modulo $d$ as columns of a matrix $H$. The
 matrix $x$ must have \typ{INT} entries, and $d$ can be an arbitrary positive
 integer. If $U$ is present, set it to a matrix such that~$AU = H$.

 \bprog
 ? A = [2,1;0,2];
 ? matimagemod(A,6,&U)
 %2 =
 [1 0]

 [0 2]

 ? U
 %3 =
 [5 1]

 [3 4]

 ? (A*U)%6
 %4 =
 [1 0]

 [0 2]
 @eprog

 \misctitle{Caveat} In general the number of columns of the Howell form is not
 the minimal number of generators of the submodule. Example:

 \bprog
 ? matimagemod([1;2],4)
 %5 =
 [2 1]

 [0 2]
 @eprog

 \misctitle{Caveat 2} In general the matrix $U$ is not invertible, even if~$A$
 and~$H$ have the same size. Example:

 \bprog
 ? matimagemod([4,1;0,4],8,&U)
 %6 =
 [2 1]

 [0 4]

 ? U
 %7 =
 [0 0]

 [2 1]
 @eprog
