Singular

D.14.1.10 arrCenter

Procedure from library `arr.lib` (see arr_lib).

Usage:
arrCenter(A); arr A

Return:
[list] L entry 0 if A not centered or entries 1, x, H, where x is any particular point of the center and H is a matrix consisting of vectors which spanning linear intersection space.
If there is exactly one solution, then H = 0.

Note:
The intersection of all hyperplanes can be expressed in forms of a linear system Ax=b, where (A|b) is the coeff. matrix of the arrange- ment, which is then solved using L-U decomposition

Example:
 ```LIB "arr.lib"; ring R = 0,(x,y,z),dp; arr A= ideal(x,y,x-y+1); // centerless arrCenter(A); ==> [1]: ==> 0 arr B= ideal(x,y,z); // center is a single point arrCenter(B); ==> [1]: ==> 1 ==> [2]: ==> _[1,1]=0 ==> _[2,1]=0 ==> _[3,1]=0 ==> [3]: ==> _[1,1]=0 arr C= ideal(x,z,x+z); // center is a line // here we get a wrong result because the matrix is simplified since A doesn't // contain any "y" the matrix (A|b) will be 3x3 only. arrCenter(C); ==> [1]: ==> 1 ==> [2]: ==> _[1,1]=0 ==> _[2,1]=0 ==> _[3,1]=0 ==> [3]: ==> _[1,1]=0 ==> _[2,1]=-1 ==> _[3,1]=0 ```