manualExt -- computes the Ext module of two modules as the core Ext function sometimes does not work properly in the negatively graded case

• Usage:

  M = manualExt(t, N, P)

• Inputs:
  • t, an integer, the homological degree
  • N, a module, a module
  • P, a module, a module
• Outputs:
  • M, a module,

i1 : R = S = QQ[X,T,Degrees=>{1,-1}];

i2 : N = S^1/ideal(X*T-1);

i3 : P = S^1;

i4 : manualExt(1, N, P)

o4 = cokernel | XT-1 |

                            1
o4 : S-module, quotient of S

i5 : Ext^1(N,P)

o5 = cokernel | XT-1 |

                            1
o5 : S-module, quotient of S