def six_hump_camelback(x):
    """
    Six-hump Camelback function (2-D).

    Global Optima: -1.031628453489877
    at (-0.08983, 0.7126) and (0.08983, -0.7126).

    Parameters
    ----------
    x : array
        2 x-values. `len(x)=2`.
        `x[i]` bound to [-5, 5] for i=1 and 2.

    Returns
    -------
    float
       Value of Six-hump Camelback function.

    """
    x1 = x[0]
    x2 = x[1]
    f = (4 - 2.1*x1**2 + x1**4/3)*x1**2 + x1*x2 + (-4 + 4*x2**2)*x2**2
    return f