docs: def dim_dtau(self, pars):
          r"""
          :math:`\frac{\partial \hat{\rho''}(\omega)}{\partial \tau} = \rho_0
          \frac{-m \omega^c c \tau^{c-1} sin(\frac{c \pi}{2} }{1 + 2 (\omega
          \tau)^c cos(\frac{c \pi}{2}) + (\omega \tau)^{2 c}} +
          \rho_0 \frac{\left[-m (\omega \tau)^c sin(\frac{c \pi}{2}
          \right] \cdot \left[ 2 \omega^c c \tau^{c-1} cos(\frac{c
          \pi}{2}) + 2 c \omega^{2 c} \tau^{2 c - 1}\right]}{\left[1 + 2 (\omega
          \tau)^c cos(\frac{c \pi}{2}) + (\omega \tau)^{2 c}\right]^2}`
          """
          self._set_parameters(pars)
          # term1
          nom1 = - self.m * np.sin(self.ang) * self.w ** self.c *\
              self.c * self.tau ** (self.c - 1)
          term1 = nom1 / self.denom
  
          # term2
          nom2 = (self.m * self.otc * np.sin(self.ang)) *\
              (2 * self.w ** self.c * self.c * self.tau ** (self.c - 1) *
               np.cos(self.ang) + 2 * self.c * self.w ** (2 * self.c) *
               self.tau ** (2 * self.c - 1))
          term2 = nom2 / self.denom ** 2
  
          result = term1 + term2
          result *= self.rho0
          return result