##### 7.7.18.0. purityFiltration

Procedure from library `purityfiltration.lib` (see purityfiltration_lib).

Usage:
  purityFiltration(S), S matrix with entries of an Auslander regular ring D

Return:
  a list T of two lists, purity filtration of the module M=D^q/D^p(S^t)

Purpose:
  the first list T[1] gives a filtration {M_i} of M,
  where the i-th entry of T[1] gives the representation matrix of M_(i-1).
  the second list T[2] gives representations of the factor Modules,
  i.e. T[2][i] gives the repr. matrix for M_(i-1)/M_i

Example:

    LIB "purityfiltration.lib";
    ring D = 0,(x1,x2,d1,d2),dp;
    def S=Weyl();
    setring S;
    int i;
    matrix R[3][3]=0,d2-d1,d2-d1,d2,-d1,-d1-d2,d1,-d1,-2*d1;
    print(R);
    ==> 0, -d1+d2,-d1+d2,
    ==> d2,-d1,   -d1-d2,
    ==> d1,-d1,   -2*d1
    list T=purityFiltration(transpose(R));
    // the purity filtration of coker(M)
    print(T[1][1]);
    ==> 0, -d1+d2,-d1+d2,
    ==> d2,-d1,   -d1-d2,
    ==> d1,-d1,   -2*d1
    print(T[1][2]);
    ==> d2,   d2,
    ==> d1-d2,0,
    ==> d2,   d1
    print(T[1][3]);
    ==> 1,0,
    ==> 0,d2,
    ==> 0,d1
    // factor modules of the filtration
    print(T[2][1]);
    ==> 0, 1,1,
    ==> -1,0,1
    print(T[2][2]);
    ==> 1,    1,
    ==> d1-d2,0
    print(T[2][3]);
    ==> 1,0,
    ==> 0,d2,
    ==> 0,d1

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