##### 7.7.3.0. center

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

Usage:
  center(D[, N]); D int, N optional int

Return:
  ideal, generated by computed elements

Purpose:
  computes subalgebra generators of the center up to degree D

Note:
  In general, one cannot compute the whole center.
  Hence, one has to specify a termination condition via arguments D and/or N.
  If D is positive, only central elements up to degree D will be found.
  If D is negative, the termination is determined by N only.
  If N is given, the computation stops if at least N elements have been found.
  Warning: if N is given and bigger than the actual number of generators,
  the procedure may not terminate.
  Current ordering must be a degree compatible well-ordering.

Example:

    LIB "central.lib";
    ring AA = 0,(x,y,z,t),dp;
    matrix D[4][4]=0;
    D[1,2]=-z;  D[1,3]=2*x;  D[2,3]=-2*y;
    def A = nc_algebra(1,D); setring A; // this algebra is U(sl_2) tensored with K[t]
    // find generators of the center of degree <= 3:
    ideal Z = center(3);
    Z;
    ==> Z[1]=t
    ==> Z[2]=4xy+z2-2z
    inCenter(Z); // check the result
    ==> 1
    // find at least one generator of the center:
    ideal Z2 = center(-1, 1);
    Z2;
    ==> Z2[1]=t
    inCenter(Z2); // check the result
    ==> 1

See also: centralizer; inCenter.

User manual for Singular version 4-0-3, 2016, generated by texi2html.