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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

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