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# scipy.sparse.linalg.lsmr

`scipy.sparse.linalg.``lsmr`(A, b, damp=0.0, atol=1e-06, btol=1e-06, conlim=100000000.0, maxiter=None, show=False)

  Iterative solver for least-squares problems.

  lsmr solves the system of linear equations `Ax = b`. If the system is inconsistent, it solves the least-squares problem `min ||b - Ax||_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).

  New in version 0.11.0.

  > Parameters : | A : {matrix, sparse matrix, ndarray, LinearOperator} Matrix A in the linear system. b : (m,) ndarray 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. atol, btol : float 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^{T} r) <= atol * norm(A) * norm(r)`. If both tolerances are 1.0e-6 (say), 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 `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. maxiter : int `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. show : bool Print iterations logs if `show=True`.
  > 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.
  >             = 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^T (b - Ax))` norma : float `norm(A)` conda : float Condition number of A. normx : float `norm(x)`

  References

  [R14] | 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. http://arxiv.org/abs/1006.0758

  [R15] | LSMR Software, http://www.stanford.edu/~clfong/lsmr.html

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