def METHOD_NAME(cos_theta):
    global _kvals, _e_ct_vals
    if _kvals is None:
        _kvals = np.logspace(-4, 1, 500)
        _e_ct_vals = np.array([_exp_cos_theta(k) for k in _kvals])
    return np.interp(cos_theta, _e_ct_vals, _kvals)