def obj(x, user_data):
    """Six-hump camelback function"""
    
    x1 = x[0]
    x2 = x[1]
    
    f = (4 - 2.1*(x1*x1) + (x1*x1*x1*x1)/3.0)*(x1*x1) + x1*x2 + (-4 + 4*(x2*x2))*(x2*x2)
    return f, 0