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D.4.3.6 homology

Procedure from library homolog.lib (see homolog_lib).

Usage:
homology(A,B,M,N);

Compute:
Let M and N be submodules of R^m and R^n presenting M'=R^m/M, N'=R^n/N (R=basering) and let A,B matrices inducing maps R^k--A-->R^m--B-->R^n. Compute a presentation R^q --H-> R^m of the module
ker(B)/im(A) := ker(M'/im(A) --B--> N'/im(BM)+im(BA)).
If B induces a map M'--B-->N' (i.e BM=0) and if R^k--A-->M'--B-->N' is a complex (i.e. BA=0) then ker(B)/im(A) is the homology of the complex

Return:
module H, a presentation of ker(B)/im(A)

Note:
homology returns a free module of rank m if ker(B)=im(A)

Example:
 
LIB "homolog.lib";
ring r;
ideal id=maxideal(4);
qring qr=std(id);
module N=maxideal(3)*freemodule(2);
module M=maxideal(2)*freemodule(2);
module B=[2x,0],[x,y],[z2,y];
module A=M;
degree(std(homology(A,B,M,N)));"";
==> // codimension = 3
==> // dimension   = 0
==> // degree      = 19
==> 
ring s=0,x,ds;
qring qs=std(x4);
module A=[x];module B=A;
module M=[x3];module N=M;
homology(A,B,M,N);
==> _[1]=gen(1)


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