def K_wall_discretization_error(dt, dx, n_c):
    """
    Best fit to error ratio of thermal conductivity for finite dt, dx, n_c.

    Parameters:
        dt: normalized timestep
        dx: normalized cell size
        n_c: particles per cell
    
    Returns:
        K_wall(dt, dx, n_c) / K(0, 0, infty)

    Rader, D. J., M. A. Gallis, J. R. Torczynski, and W. Wagner.
    “Direct Simulation Monte Carlo Convergence Behavior of
    the Hard-Sphere-Gas Thermal Conductivity for Fourier Heat Flow.”
    Physics of Fluids 18, no. 7 (July 2006): 077102.
    https://doi.org/10.1063/1.2213640.

    Equation (23)
    """
    ratio = (1.0001 + 0.0287 * dt ** 2 + 0.0405 * dx ** 2
                   - 0.0009 * dx ** 4 - 0.016 * dt ** 2 * dx ** 2
                   + 0.0081 * dt ** 4 * dx **2
                   + (1 / n_c) * (-0.083 + 1.16 * dx
                                         - 0.220 * dx ** 2
                                         + 1.56 * dt ** 2 
                                         - 2.55 * dt ** 2 * dx
                                         + 1.14 * dt ** 2 * dx ** 2
                                         - 0.92 * dt ** 3
                                         + 1.91 * dt ** 3 * dx 
                                         - 0.94 * dt ** 3 * dx ** 2
                                         )
                   + (1 / n_c ** 2) * 0.95 * dt ** 2
                   )
    return ratio