def simple_mean_function(max_iters=100, optimize=True, plot=True):
    """
    The simplest possible mean function. No parameters, just a simple Sinusoid.
    """
    #create  simple mean function
    mf = GPy.core.Mapping(1,1)
    mf.f = np.sin
    mf.update_gradients = lambda a,b: None

    X = np.linspace(0,10,50).reshape(-1,1)
    Y = np.sin(X) + 0.5*np.cos(3*X) + 0.1*np.random.randn(*X.shape)

    k =GPy.kern.RBF(1)
    lik = GPy.likelihoods.Gaussian()
    m = GPy.core.GP(X, Y, kernel=k, likelihood=lik, mean_function=mf)
    if optimize:
        m.optimize(max_iters=max_iters)
    if plot:
        m.plot(plot_limits=(-10,15))
    return m