scipy.sparse.linalg.lsmr¶

scipy.sparse.linalg.
lsmr
(A, b, damp=0.0, atol=1e06, btol=1e06, conlim=100000000.0, maxiter=None, show=False, x0=None)[source]¶ Iterative solver for leastsquares problems.
lsmr solves the system of linear equations
Ax = b
. If the system is inconsistent, it solves the leastsquares problemmin b  Ax_2
. A is a rectangular matrix of dimension mbyn, 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{matrix, sparse matrix, ndarray, LinearOperator}
Matrix A in the linear system. Alternatively,
A
can be a linear operator which can produceAx
andA^H x
using, e.g.,scipy.sparse.linalg.LinearOperator
. barray_like, shape (m,)
Vector b in the linear system.
 dampfloat
Damping factor for regularized leastsquares.
lsmr
solves the regularized leastsquares 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, btolfloat, optional
Stopping tolerances.
lsmr
continues iterations until a certain backward error estimate is smaller than some quantity depending on atol and btol. Letr = b  Ax
be the residual vector for the current approximate solutionx
. IfAx = b
seems to be consistent,lsmr
terminates whennorm(r) <= atol * norm(A) * norm(x) + btol * norm(b)
. Otherwise, lsmr terminates whennorm(A^H r) <= atol * norm(A) * norm(r)
. If both tolerances are 1.0e6 (say), the finalnorm(r)
should be accurate to about 6 digits. (The final x will usually have fewer correct digits, depending oncond(A)
and the size of LAMBDA.) If atol or btol is None, a default value of 1.0e6 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 = 1e7. This prevents the algorithm from doing unnecessary work beyond the uncertainty of the input data. conlimfloat, optional
lsmr
terminates if an estimate ofcond(A)
exceeds conlim. For compatible systemsAx = b
, conlim could be as large as 1.0e+12 (say). For leastsquares 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 settingatol = btol = conlim = 0
, but the number of iterations may then be excessive. maxiterint, optional
lsmr
terminates if the number of iterations reaches maxiter. The default ismaxiter = min(m, n)
. For illconditioned systems, a larger value of maxiter may be needed. showbool, optional
Print iterations logs if
show=True
. x0array_like, shape (n,), optional
Initial guess of x, if None zeros are used.
New in version 1.0.0.
 Returns
 ——
 xndarray of float
Leastsquare solution returned.
 istopint
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 leastsquares 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.
 itnint
Number of iterations used.
 normrfloat
norm(bAx)
 normarfloat
norm(A^H (b  Ax))
 normafloat
norm(A)
 condafloat
Condition number of A.
 normxfloat
norm(x)
Notes
New in version 0.11.0.
References
 1
D. C.L. Fong and M. A. Saunders, “LSMR: An iterative algorithm for sparse leastsquares problems”, SIAM J. Sci. Comput., vol. 33, pp. 29502971, 2011. https://arxiv.org/abs/1006.0758
 2
LSMR Software, https://web.stanford.edu/group/SOL/software/lsmr/
Examples
>>> from scipy.sparse import csc_matrix >>> from scipy.sparse.linalg import lsmr >>> A = csc_matrix([[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 nontrivial 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 4.440892098500627e16
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 leastsquares problem. normr contains the minimal distance that was found.