code: def calc_delta_c(c200):
      
      top = (200. / 3.) * c200**3.
      bottom = np.log(1. + c200) - (c200 / (1. + c200))
      return (top / bottom)
docs: Calculate characteristic overdensity from concentration.
  
      Parameters
      ----------
      c200 : ndarray or float
          Cluster concentration parameter.
  
      Returns
      ----------
      ndarray or float
          Cluster characteristic overdensity, of same type as c200.