Singular Manual: alg_kernel

| #### D.4.2.5 alg_kernel Procedure from library `algebra.lib` (see algebra_lib). Usage: alg_kernel(phi,pr[,s,c]); phi map to basering, pr preimage ring, s string (name of kernel in pr), c integer. Return: a string, the kernel of phi as string. If, moreover, a string s is given, the algorithm creates, in the preimage ring pr the kernel of phi with name s. Three different algorithms are used depending on c = 1,2,3. If c is not given or c=0, a heuristically best method is chosen. (algorithm 1 uses the preimage command) Note: Since the kernel of phi lives in pr, it cannot be returned to the basering. If s is given, the user has access to it in pr via s. The basering may be a quotient ring. Example:
    LIB "algebra.lib";
    ring r = 0,(a,b,c),ds;
    ring s = 0,(x,y,z,u,v,w),dp;
    ideal I = x-w,u2w+1,yz-v;
    map phi = r,I;                // a map from r to s:
    alg_kernel(phi,r);            // a,b,c ---> x-w,u2w+1,yz-v
    ==> 0
    ring S = 0,(a,b,c),ds;
    ring R = 0,(x,y,z),dp;
    qring Q = std(x-y);
    ideal i = x, y, x2-y3;
    map phi = S,i;                 // a map to a quotient ring
    alg_kernel(phi,S,"ker",3);     // uses algorithm 3
    ==> a-b,b^3-b^2+c
    setring S;                     // you have access to kernel in preimage
    ker;
    ==> ker[1]=a-b
    ==> ker[2]=c-b2+b3

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