p4], np.dot(q4, r4))
    True
    >>> q4.shape, r4.shape, p4.shape
    ((9, 9), (9, 6), (6,))

    >>> q5, r5, p5 = linalg.qr(a, mode='economic', pivoting=True)
    >>> q5.shape, r5.shape, p5.shape
    ((9, 6), (6, 6), (6,))

    )