perator output vector if True.
structure : str, optional
    A string describing the structure of all operators.
    Only `upper_triangular` is currently supported.

Returns
-------
est : float
    An underestimate of the 1-norm of the sparse arrays.
v : ndarray, optional
    The vector such that ||Av||_1 == est*||v||_1.
    It can be thought of as an input to the linear operator
    that gives an output with particularly large norm.
w : ndarray, optional
    The vector Av which has relatively large 1-norm.
    It can be thought of as an output of the linear operator
    that is relatively large in norm compared to the input.

r@