x||_2``.
``A`` is a rectangular matrix of dimension m-by-n, where all cases are
allowed: m = n, m > n, or m < n. ``b`` is a vector of length m.
The matrix A may be dense or sparse (usually sparse).

Parameters
----------
A : {sparse array, ndarray, LinearOperator}
    Matrix A in the linear system.
    Alternatively, ``A`` can be a linear operator which can
    produce ``Ax`` and ``A^H x`` using, e.g.,
    ``scipy.sparse.linalg.LinearOperator``.
b : array_like, shape (m,)
    Vector ``b`` in the linear system.
damp : float
    Damping factor for regularized least-squares. `lsmr` solves
    the regularized least-squares problem::

     min ||(b) - (  A   )x||
         ||(0)   (damp*I) ||_2

    where damp is a scalar.  If damp is None or 0, the system
    is solved without regularization. Default is 0.
atol, btol : float, optional
    Stopping tolerances. `lsmr` continues iterations until a
    certain backward error estimate is smaller than some quantity
    depending on atol and btol.  Let ``r = b - Ax`` be the
    residual vector for the current approximate solution ``x``.
    If ``Ax = b`` seems to be consistent, `lsmr` terminates
    when ``norm(r) <= atol * norm(A) * norm(x) + btol * norm(b)``.
    Otherwise, `lsmr` terminates when ``norm(A^H r) <=
    atol * norm(A) * norm(r)``.  If both tolerances are 1.0e-6 (default),
    the final ``norm(r)`` should be accurate to about 6
    digits. (The final ``x`` will usually have fewer correct digits,
    depending on ``cond(A)`` and the size of LAMBDA.)  If `atol`
    or `btol` is None, a default value of 1.0e-6 will be used.
    Ideally, they should be estimates of the relative error in the
    entries of ``A`` and ``b`` respectively.  For example, if the entries
    of ``A`` have 7 correct digits, set ``atol = 1e-7``. This prevents
    the algorithm from doing unnecessary work beyond the
    uncertainty of the input data.
conlim : float, optional
    `lsmr` terminates if an estimate of ``cond(A)`` exceeds
    `conlim`.  For compatible systems ``Ax = b``, conlim could be
    as large as 1.0e+12 (say).  For least-squares problems,
    `conlim` should be less than 1.0e+8. If `conlim` is None, the
    default value is 1e+8.  Maximum precision can be obtained by
    setting ``atol = btol = conlim = 0``, but the number of
    iterations may then be excessive. Default is 1e8.
maxiter : int, optional
    `lsmr` terminates if the number of iterations reaches
    `maxiter`.  The default is ``maxiter = min(m, n)``.  For
    ill-conditioned systems, a larger value of `maxiter` may be
    needed. Default is False.
show : bool, optional
    Print iterations logs if ``show=True``. Default is False.
x0 : array_like, shape (n,), optional
    Initial guess of ``x``, if None zeros are used. Default is None.

    .. versionadded:: 1.0.0

Returns
-------
x : ndarray of float
    Least-square solution returned.
istop : int
    istop gives the reason for stopping::

      istop   = 0 means x=0 is a solution.  If x0 was given, then x=x0 is a
                  solution.
              = 1 means x is an approximate solution to A@x = B,
                  according to atol and btol.
              = 2 means x approximately solves the least-squares problem
                  according to atol.
              = 3 means COND(A) seems to be greater than CONLIM.
              = 4 is the same as 1 with atol = btol = eps (machine
                  precision)
              = 5 is the same as 2 with atol = eps.
              = 6 is the same as 3 with CONLIM = 1/eps.
              = 7 means ITN reached maxiter before the other stopping
                  conditions were satisfied.

itn : int
    Number of iterations used.
normr : float
    ``norm(b-Ax)``
normar : float
    ``norm(A^H (b - Ax))``
norma : float
    ``norm(A)``
conda : float
    Condition number of A.
normx : float
    ``norm(x)``

Notes
-----

.. versionadded:: 0.11.0

References
----------
.. [1] D. C.-L. Fong and M. A. Saunders,
       "LSMR: An iterative algorithm for sparse least-squares problems",
       SIAM J. Sci. Comput., vol. 33, pp. 2950-2971, 2011.
       :arxiv:`1006.0758`
.. [2] LSMR Software, https://web.stanford.edu/group/SOL/software/lsmr/

Examples
--------
>>> import numpy as np
>>> from scipy.sparse import csc_array
>>> from scipy.sparse.linalg import lsmr
>>> A = csc_array([[1., 0.], [1., 1.], [0., 1.]], dtype=float)

The first example has the trivial solution ``[0, 0]``

>>> b = np.array([0., 0., 0.], dtype=float)
>>> x, istop, itn, normr = lsmr(A, b)[:4]
>>> istop
0
>>> x
array([0., 0.])

The stopping code ``istop=0`` returned indicates that a vector of zeros was
found as a solution. The returned solution `x` indeed contains
``[0., 0.]``. The next example has a non-trivial solution:

>>> b = np.array([1., 0., -1.], dtype=float)
>>> x, istop, itn, normr = lsmr(A, b)[:4]
>>> istop
1
>>> x
array([ 1., -1.])
>>> itn
1
>>> normr
4.440892098500627e-16

As indicated by ``istop=1``, `lsmr` found a solution obeying the tolerance
limits. The given solution ``[1., -1.]`` obviously solves the equation. The
remaining return values include information about the number of iterations
(`itn=1`) and the remaining difference of left and right side of the solved
equation.
The final example demonstrates the behavior in the case where there is no
solution for the equation:

>>> b = np.array([1., 0.01, -1.], dtype=float)
>>> x, istop, itn, normr = lsmr(A, b)[:4]
>>> istop
2
>>> x
array([ 1.00333333, -0.99666667])
>>> A.dot(x)-b
array([ 0.00333333, -0.00333333,  0.00333333])
>>> normr
0.005773502691896255

`istop` indicates that the system is inconsistent and thus `x` is rather an
approximate solution to the corresponding least-squares problem. `normr`
contains the minimal distance that was found.
