def viscosity_index(nu_40, nu_100, rounding=False):
nu_40, nu_100 = nu_40*<NUM_LIT>, nu_100*<NUM_LIT>  <EOL>if nu_100 < <NUM_LIT:2>:<EOL><INDENT>return None  <EOL><DEDENT>elif nu_100 < <NUM_LIT>:<EOL><INDENT>L = np.interp(nu_100, VI_nus, VI_Ls)<EOL>H = np.interp(nu_100, VI_nus, VI_Hs)<EOL><DEDENT>else:<EOL><INDENT>L = <NUM_LIT>*nu_100**<NUM_LIT:2> + <NUM_LIT>*nu_100 - <NUM_LIT><EOL>H = <NUM_LIT>*nu_100**<NUM_LIT:2> + <NUM_LIT>*nu_100 - <NUM_LIT><EOL><DEDENT>if nu_40 > H:<EOL><INDENT>VI = (L-nu_40)/(L-H)*<NUM_LIT:100><EOL><DEDENT>else:<EOL><INDENT>N = (log(H) - log(nu_40))/log(nu_100)<EOL>VI = (<NUM_LIT:10>**N-<NUM_LIT:1>)/<NUM_LIT> + <NUM_LIT:100><EOL><DEDENT>if rounding:<EOL><INDENT>VI = _round_whole_even(VI)<EOL><DEDENT>return VI<EOL>
r'''Calculates the viscosity index of a liquid. Requires dynamic viscosity
    of a liquid at 40°C and 100°C. Value may either be returned with or
    without rounding. Rounding is performed per the standard.

    if nu_100 < 70:

    .. math::
        L, H = interp(nu_100)

    else:

    .. math::
        L = 0.8353\nu_{100}^2 + 14.67\nu_{100} - 216

        H = 0.1684\nu_{100}^2 + 11.85\nu_{100} - 97

    if nu_40 > H:

    .. math::
        VI = \frac{L-nu_{40}}{L-H}\cdot 100

    else:

    .. math::
        N = \frac{\log(H) - \log(\nu_{40})}{\log (\nu_{100})}

         VI = \frac{10^N-1}{0.00715} + 100

    Parameters
    ----------
    nu_40 : float
        Dynamic viscosity of fluid at 40°C, [m^2/s]
    nu_100 : float
        Dynamic viscosity of fluid at 100°C, [m^2/s]
    rounding : bool, optional
        Whether to round the value or not.

    Returns
    -------
    VI: float
        Viscosity index [-]

    Notes
    -----
    VI is undefined for nu_100 under 2 mm^2/s. None is returned if this is the
    case. Internal units are mm^2/s. Higher values of viscosity index suggest
    a lesser decrease in kinematic viscosity as temperature increases.

    Note that viscosity is a pressure-dependent property, and that the 
    viscosity index is defined for a fluid at whatever pressure it is at.
    The viscosity index is thus also a function of pressure.

    Examples
    --------
    >>> viscosity_index(73.3E-6, 8.86E-6, rounding=True)
    92

    References
    ----------
    .. [1] ASTM D2270-10(2016) Standard Practice for Calculating Viscosity
       Index from Kinematic Viscosity at 40 °C and 100 °C, ASTM International,
       West Conshohocken, PA, 2016, http://dx.doi.org/10.1520/D2270-10R16