code: public static void root(ComplexPolar_F64 a , int N , int k , ComplexPolar_F64 result )
      {
          result.r = Math.pow(a.r,1.0/N);
          result.theta = (a.theta + 2.0*k* UtilEjml.PI)/N;
      }
docs: Computes the N<sup>th</sup> root of a complex number in polar notation.  There are
  N distinct N<sup>th</sup> roots.
  
  @param a Complex number
  @param N The root's magnitude
  @param k Specifies which root.  0 &le; k &lt; N
  @param result Computed root