def dim_dtau(self, pars):
        r"""
        :math:Add formula
        """
        self._set_parameters(pars)
        # term 1
        num1 = -self.m * (self.w ** self.c) * self.c\
            * (self.tau ** (self.c - 1)) * np.sin(self.ang)
        term1 = self.sigmai * num1 / self.denom

        # term 2
        num2a = -self.m * self.otc * np.sin(self.ang)
        num2b = 2 * (self.w ** 2.0) * self.c * (self.tau ** (self.c - 1)) *\
            np.cos(self.ang)
        num2c = 2 * self.c * (self.w ** (self.c * 2)) *\
            (self.tau ** (2 * self.c - 1))
        term2 = self.sigma0 * num2a * (num2b + num2c) / (self.denom ** 2)

        result = term1 + term2

        return result