| LIB "homolog.lib";
int p = printlevel;
printlevel = 1;
ring r = 0,(x,y),dp;
ideal i = x2-y3;
ideal j = x2-y5;
list E = Ext(0..2,i,j); // Ext^k(r/i,r/j) for k=0,1,2 over r
→ // Computing Ext^0 (help Ext; gives an explanation):
→ // Let 0<--coker(M)<--F0<--F1<--F2<--... be a resolution of coker(M),
→ // and 0<--coker(N)<--G0<--G1 a presentation of coker(N),
→ // then Hom(F0,G0)-->Hom(F1,G0) is given by:
→ y3-x2
→ // and Hom(F-1,G0) + Hom(F0,G1)-->Hom(F0,G0) is given by:
→ 0,-y5+x2
→
→ // dimension of Ext^0: -1
→
→ // Computing Ext^1 (help Ext; gives an explanation):
→ // Let 0<--coker(M)<--F0<--F1<--F2<--... be a resolution of coker(M),
→ // and 0<--coker(N)<--G0<--G1 a presentation of coker(N),
→ // then Hom(F1,G0)-->Hom(F2,G0) is given by:
→ 0
→ // and Hom(F0,G0) + Hom(F1,G1)-->Hom(F1,G0) is given by:
→ y3-x2,-y5+x2
→
→ // dimension of Ext^1: 0
→ // vdim of Ext^1: 10
→
→ // Computing Ext^2 (help Ext; gives an explanation):
→ // Let 0<--coker(M)<--F0<--F1<--F2<--... be a resolution of coker(M),
→ // and 0<--coker(N)<--G0<--G1 a presentation of coker(N),
→ // then Hom(F2,G0)-->Hom(F3,G0) is given by:
→ 1
→ // and Hom(F1,G0) + Hom(F2,G1)-->Hom(F2,G0) is given by:
→ 0,-y5+x2
→
→ // dimension of Ext^2: -1
→
qring R = std(i);
ideal j = fetch(r,j);
module M = [-x,y],[-y2,x];
printlevel = 2;
module E1 = Ext(1,M,j); // Ext^1(R^2/M,R/j) over R=r/i
→ // Computing Ext^1 (help Ext; gives an explanation):
→ // Let 0<--coker(M)<--F0<--F1<--F2<--... be a resolution of coker(M),
→ // and 0<--coker(N)<--G0<--G1 a presentation of coker(N),
→ // then Hom(F1,G0)-->Hom(F2,G0) is given by:
→ x, -y,
→ y2,-x
→ // and Hom(F0,G0) + Hom(F1,G1)-->Hom(F1,G0) is given by:
→ x, -y,-y5+x2,0,
→ y2,-x,0, -y5+x2
→
→ // dimension of Ext^1: -1
→
list l = Ext(4,M,M,1); // Ext^4(R^2/M,R^2/M) over R=r/i
→ // Computing Ext^4 (help Ext; gives an explanation):
→ // Let 0<--coker(M)<--F0<--F1<--F2<--... be a resolution of coker(M),
→ // and 0<--coker(N)<--G0<--G1 a presentation of coker(N),
→ // then Hom(F4,G0)-->Hom(F5,G0) is given by:
→ x, -y,0, 0,
→ y2,-x,0, 0,
→ 0, 0, x, -y,
→ 0, 0, y2,-x
→ // and Hom(F3,G0) + Hom(F4,G1)-->Hom(F4,G0) is given by:
→ x, -y,0, 0, -x,0, -y2,0,
→ y2,-x,0, 0, 0, -x,0, -y2,
→ 0, 0, x, -y,y, 0, x, 0,
→ 0, 0, y2,-x,0, y, 0, x
→
→ // dimension of Ext^4: 0
→ // vdim of Ext^4: 2
→
→ // columns of matrix are kbase of Ext^4 in Hom(F4,G0)
→ 1,0,
→ 0,y,
→ 0,1,
→ 1,0
→
→ // element 1 of kbase of Ext^4 in Hom(F4,G0)
→ // as matrix: F4-->G0
→ 1,0,
→ 0,1
→ // element 2 of kbase of Ext^4 in Hom(F4,G0)
→ // as matrix: F4-->G0
→ 0,y,
→ 1,0
→
printlevel = p;
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