def _compute_f3(self, C, mag):
        """
        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
        """
        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']
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