def nu_i(n_i, n_n, A):
    """approximate calculation of ion collision frequency from Kelley 89

    Parameters
    ----------
    n_i : (float)
        ion density cm-3
    n_n : (float)
        neutral density cm-3
    A : (int)
        mean neutral molecular mass in atomic mass units
    """
    return 2.6 * 10**(-9) * (n_i + n_n) * A**(-1/2)