Description
The set of Dhomomorphisms between two holonomic modules
M and
N is a finitedimensional vector space over the ground field. Since a homomorphism is defined by where it sends a set of generators, the output of this command is a list of matrices whose columns correspond to the images of the generators of
M. Here the generators of
M are determined from its presentation by generators and relations.
The procedure calls Drestriction, which uses w if specified.
The algorithm used appears in the paper 'Computing homomorphisms between holonomic Dmodules' by TsaiWalther(2000). The method is to combine isomorphisms of Bjork and Kashiwara with the restriction algorithm.
i1 : W = QQ[x, D, WeylAlgebra=>{x=>D}]
o1 = W
o1 : PolynomialRing, 1 differential variable(s)

i2 : M = W^1/ideal(D1)
o2 = cokernel  D1 
1
o2 : Wmodule, quotient of W

i3 : N = W^1/ideal((D1)^2)
o3 = cokernel  D22D+1 
1
o3 : Wmodule, quotient of W

i4 : DHom(M,N)
o4 = { xD+x+1 ,  D+1 }
o4 : List
