- Usage:
`DExt(M,N), DExt(M,N,w)`

- Outputs:
- a hash table, the
`Ext`groups between holonomic D-modules*M*and*N*

- a hash table, the

The Ext groups between D-modules *M* and *N* are the derived functors of Hom, and are finite-dimensional vector spaces over the ground field when *M* and *N* are holonomic.

The procedure calls Drestriction, which uses *w* if specified.

The algorithm used appears in the paper 'Polynomial and rational solutions of holonomic systems' by Oaku-Takayama-Tsai (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 |

i2 : M = W^1/ideal(x*(D-1)) o2 = cokernel | xD-x | 1 o2 : W-module, quotient of W |

i3 : N = W^1/ideal((D-1)^2) o3 = cokernel | D2-2D+1 | 1 o3 : W-module, quotient of W |

i4 : DExt(M,N) 2 o4 = HashTable{0 => QQ } 2 1 => QQ o4 : HashTable |

Input modules M, N should be holonomic.Does not yet compute explicit reprentations of Ext groups such as Yoneda representation.

- DHom -- D-homomorphisms between holonomic D-modules
- Drestriction -- restriction modules of a D-module

- DExt(Module,Module)
- DExt(Module,Module,List)