Function: core
Section: number_theoretical
C-Name: core0
Prototype: GD0,L,
Help: core(n,{flag=0}): unique (positive of negative) squarefree integer d
 dividing n such that n/d is a square. If (optional) flag is non-null, output
 the two-component row vector [d,f], where d is the unique squarefree integer
 dividing n such that n/d=f^2 is a square

