##### 7.7.3.0. centralizeSet

Procedure from library `central.lib` (see central_lib).

Usage:
  centralizeSet( F, V ); F, V ideals

Input:
  F, V finite sets of elements of the base algebra

Return:
  ideal, generated by computed elements

Purpose:
  computes a vector space basis of the centralizer of the set F in the vector space generated by V over the ground field

Example:

    LIB "central.lib";
    ring A = 0,(e(1..4)),dp;
    matrix D[4][4]=0;
    D[2,4] = -e(1);
    D[3,4] = -e(2);
    // This is A_4_1 - the first real Lie algebra of dimension 4.
    def A_4_1 = nc_algebra(1,D); setring A_4_1;
    ideal F = variablesSorted(); F;
    ==> F[1]=e(1)
    ==> F[2]=e(4)
    ==> F[3]=e(3)
    ==> F[4]=e(2)
    // the center of A_4_1 is generated by
    // e(1) and -1/2*e(2)^2+e(1)*e(3)
    // therefore one may consider computing it in the following way:
    // 1. Compute a PBW basis consisting of
    //    monomials with exponent <= (1,2,1,0)
    ideal V = PBW_maxMonom( e(1) * e(2)^ 2 * e(3) );
    // 2. Compute the centralizer of F within the vector space
    //    spanned by these monomials:
    ideal C = centralizeSet( F, V ); C;
    ==> C[1]=e(1)
    ==> C[2]=e(2)^2-2*e(1)*e(3)
    inCenter(C); // check the result
    ==> 1

See also: centralizer; centralizerVS; inCentralizer.

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