code: def _compute_f3(self, C, mag):
          
          if mag <= 5.8:
              return C['a5']
          elif 5.8 < mag < C['c1']:
              return (
                  C['a5'] +
                  (C['a6'] - C['a5']) * (mag - 5.8) / (C['c1'] - 5.8)
              )
          else:
              return C['a6']
docs: Compute f3 term (eq.6, page 106)
  
          NOTE: In the original manuscript, for the case 5.8 < mag < c1,
          the term in the numerator '(mag - 5.8)' is missing, while is
          present in the software used for creating the verification tables