The array that defines the block diagonal structure returned by
        _ldl_sanitize_ipiv().
    lower : bool, optional
        The boolean to switch between lower and upper triangular structure.

    Returns
    -------
    lu : ndarray
        The square outer factor which satisfies the L * D * L.T = A
    perm : ndarray
        The permutation vector that brings the lu to the triangular form

    Notes
    -----
    Note that the original argument "lu" is overwritten.

    r