docs: def sbesselj_sum(z, N):
      """Tests the Spherical Bessel function jn using the sum:
  
          Inf
          sum  (2*n+1) * jn(z)**2 = 1
          n=0
  
  
          z:  The argument.
          N:  Large N value that the sum runs too.
  
      Note that the sum only converges to 1 for large N value (i.e. N >> z).
  
      The routine returns the relative error of the assumption.
      """
  
      b = sbesselj(z, N)
      vvv = 2.0 * np.array(range(0, N), dtype=np.float64) + 1.0
      sm = np.sum(np.sort(vvv * (b ** 2)))
      return np.abs((sm - 1.0) / sm) + np.spacing(1)