def varigreen(b2, b3, b4):
    """
    Visible Atmospherically Resistant Index Green \
    (Gitelson et al. 2001a).

    .. math:: VARIGREEN = (b3 - b4)/(b3 + b4 - b2)

    :param b2: Blue.
    :type b2: numpy.ndarray or float
    :param b3: Green.
    :type b3: numpy.ndarray or float
    :param b4: Red.
    :type b4: numpy.ndarray or float

    :returns VARIGREEN: Index value

    .. Tip::

        Gitelson, A., Merzlyak, M. N., Chivkunova, O. B. 2001a. \
        Optical properties and nondestructive estimation of anthocyanin \
        content in plant leaves. Photochemistry and Photobiology, 74,38-45. \
        doi:10.1562/0031-8655(2001)074<0038:opaneo>2.0.co;2.
    """

    VARIGREEN = (b3 - b4)/(b3 + b4 - b2)
    return VARIGREEN