def PCA_reduce(X, Q):
    """
    A helpful function for linearly reducing the dimensionality of the data X
    to Q.
    :param X: data array of size N (number of points) x D (dimensions)
    :param Q: Number of latent dimensions, Q < D
    :return: PCA projection array of size N x Q.
    """
    assert Q <= X.shape[1], 'Cannot have more latent dimensions than observed'
    evals, evecs = np.linalg.eigh(np.cov(X.T))
    W = evecs[:, -Q:]
    return (X - X.mean(0)).dot(W)