docs: def R_cylinder(Di, Do, k, L):
      r'''Returns the thermal resistance `R` of a cylinder of constant thermal
      conductivity `k`, of inner and outer diameter `Di` and `Do`, and with a
      length `L`.
  
      .. math::
          (hA)_{\text{cylinder}}=\frac{k}{\ln(D_o/D_i)} \cdot 2\pi L\\
          R_{\text{cylinder}}=\frac{1}{(hA)_{\text{cylinder}}}=
          \frac{\ln(D_o/D_i)}{2\pi Lk}
  
      Parameters
      ----------
      Di : float
          Inner diameter of the cylinder, [m]
      Do : float
          Outer diameter of the cylinder, [m]
      k : float
          Thermal conductivity of the cylinder, [W/m/K]
      L : float
          Length of the cylinder, [m]
  
      Returns
      -------
      R : float
          Thermal resistance [K/W]
  
      Examples
      --------
      >>> R_cylinder(0.9, 1., 20, 10)
      8.38432343682705e-05
  
      References
      ----------
      .. [1] Bergman, Theodore L., Adrienne S. Lavine, Frank P. Incropera, and
         David P. DeWitt. Introduction to Heat Transfer. 6E. Hoboken, NJ:
         Wiley, 2011.
      '''
      hA = k*2*pi*L/log(Do/Di)
      return 1./hA