float, optional
        Parameter for curvature condition rule.
    amax, amin : float, optional
        Maximum and minimum step size
    xtol : float, optional
        Relative tolerance for an acceptable step.

    Returns
    -------
    alpha : float
        Step size, or None if no suitable step was found
    phi : float
        Value of `phi` at the new point `alpha`
    phi0 : float
        Value of `phi` at `alpha=0`

    Notes
    -----
    Uses routine DCSRCH from MINPACK.

    N