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from __future__ import division, print_function, absolute_import
from numpy import sqrt, inner, finfo, zeros
from numpy.linalg import norm
from .utils import make_system
from .iterative import set_docstring
__all__ = ['minres']
header = \
"""Use MINimum RESidual iteration to solve Ax=b
MINRES minimizes norm(A*x - b) for a real symmetric matrix A. Unlike
the Conjugate Gradient method, A can be indefinite or singular.
If shift != 0 then the method solves (A - shift*I)x = b
"""
Ainfo = "The real symmetric N-by-N matrix of the linear system"
footer = \
"""
Notes
-----
THIS FUNCTION IS EXPERIMENTAL AND SUBJECT TO CHANGE!
References
----------
Solution of sparse indefinite systems of linear equations,
C. C. Paige and M. A. Saunders (1975),
SIAM J. Numer. Anal. 12(4), pp. 617-629.
http://www.stanford.edu/group/SOL/software/minres.html
This file is a translation of the following MATLAB implementation:
http://www.stanford.edu/group/SOL/software/minres/matlab/
"""
@set_docstring(header,
Ainfo,
footer)
def minres(A, b, x0=None, shift=0.0, tol=1e-5, maxiter=None, xtype=None,
M=None, callback=None, show=False, check=False):
A,M,x,b,postprocess = make_system(A,M,x0,b,xtype)
matvec = A.matvec
psolve = M.matvec
first = 'Enter minres. '
last = 'Exit minres. '
n = A.shape[0]
if maxiter is None:
maxiter = 5 * n
msg = [' beta2 = 0. If M = I, b and x are eigenvectors ', # -1
' beta1 = 0. The exact solution is x = 0 ', # 0
' A solution to Ax = b was found, given rtol ', # 1
' A least-squares solution was found, given rtol ', # 2
' Reasonable accuracy achieved, given eps ', # 3
' x has converged to an eigenvector ', # 4
' acond has exceeded 0.1/eps ', # 5
' The iteration limit was reached ', # 6
' A does not define a symmetric matrix ', # 7
' M does not define a symmetric matrix ', # 8
' M does not define a pos-def preconditioner '] # 9
if show:
print(first + 'Solution of symmetric Ax = b')
print(first + 'n = %3g shift = %23.14e' % (n,shift))
print(first + 'itnlim = %3g rtol = %11.2e' % (maxiter,tol))
print()
istop = 0
itn = 0
Anorm = 0
Acond = 0
rnorm = 0
ynorm = 0
xtype = x.dtype
eps = finfo(xtype).eps
x = zeros(n, dtype=xtype)
# Set up y and v for the first Lanczos vector v1.
# y = beta1 P' v1, where P = C**(-1).
# v is really P' v1.
y = b
r1 = b
y = psolve(b)
beta1 = inner(b,y)
if beta1 < 0:
raise ValueError('indefinite preconditioner')
elif beta1 == 0:
return (postprocess(x), 0)
beta1 = sqrt(beta1)
if check:
# are these too strict?
# see if A is symmetric
w = matvec(y)
r2 = matvec(w)
s = inner(w,w)
t = inner(y,r2)
z = abs(s - t)
epsa = (s + eps) * eps**(1.0/3.0)
if z > epsa:
raise ValueError('non-symmetric matrix')
# see if M is symmetric
r2 = psolve(y)
s = inner(y,y)
t = inner(r1,r2)
z = abs(s - t)
epsa = (s + eps) * eps**(1.0/3.0)
if z > epsa:
raise ValueError('non-symmetric preconditioner')
# Initialize other quantities
oldb = 0
beta = beta1
dbar = 0
epsln = 0
qrnorm = beta1
phibar = beta1
rhs1 = beta1
rhs2 = 0
tnorm2 = 0
ynorm2 = 0
cs = -1
sn = 0
w = zeros(n, dtype=xtype)
w2 = zeros(n, dtype=xtype)
r2 = r1
if show:
print()
print()
print(' Itn x(1) Compatible LS norm(A) cond(A) gbar/|A|')
while itn < maxiter:
itn += 1
s = 1.0/beta
v = s*y
y = matvec(v)
y = y - shift * v
if itn >= 2:
y = y - (beta/oldb)*r1
alfa = inner(v,y)
y = y - (alfa/beta)*r2
r1 = r2
r2 = y
y = psolve(r2)
oldb = beta
beta = inner(r2,y)
if beta < 0:
raise ValueError('non-symmetric matrix')
beta = sqrt(beta)
tnorm2 += alfa**2 + oldb**2 + beta**2
if itn == 1:
if beta/beta1 <= 10*eps:
istop = -1 # Terminate later
# tnorm2 = alfa**2 ??
gmax = abs(alfa)
gmin = gmax
# Apply previous rotation Qk-1 to get
# [deltak epslnk+1] = [cs sn][dbark 0 ]
# [gbar k dbar k+1] [sn -cs][alfak betak+1].
oldeps = epsln
delta = cs * dbar + sn * alfa # delta1 = 0 deltak
gbar = sn * dbar - cs * alfa # gbar 1 = alfa1 gbar k
epsln = sn * beta # epsln2 = 0 epslnk+1
dbar = - cs * beta # dbar 2 = beta2 dbar k+1
root = norm([gbar, dbar])
Arnorm = phibar * root
# Compute the next plane rotation Qk
gamma = norm([gbar, beta]) # gammak
gamma = max(gamma, eps)
cs = gbar / gamma # ck
sn = beta / gamma # sk
phi = cs * phibar # phik
phibar = sn * phibar # phibark+1
# Update x.
denom = 1.0/gamma
w1 = w2
w2 = w
w = (v - oldeps*w1 - delta*w2) * denom
x = x + phi*w
# Go round again.
gmax = max(gmax, gamma)
gmin = min(gmin, gamma)
z = rhs1 / gamma
ynorm2 = z**2 + ynorm2
rhs1 = rhs2 - delta*z
rhs2 = - epsln*z
# Estimate various norms and test for convergence.
Anorm = sqrt(tnorm2)
ynorm = sqrt(ynorm2)
epsa = Anorm * eps
epsx = Anorm * ynorm * eps
epsr = Anorm * ynorm * tol
diag = gbar
if diag == 0:
diag = epsa
qrnorm = phibar
rnorm = qrnorm
test1 = rnorm / (Anorm*ynorm) # ||r|| / (||A|| ||x||)
test2 = root / Anorm # ||Ar|| / (||A|| ||r||)
# Estimate cond(A).
# In this version we look at the diagonals of R in the
# factorization of the lower Hessenberg matrix, Q * H = R,
# where H is the tridiagonal matrix from Lanczos with one
# extra row, beta(k+1) e_k^T.
Acond = gmax/gmin
# See if any of the stopping criteria are satisfied.
# In rare cases, istop is already -1 from above (Abar = const*I).
if istop == 0:
t1 = 1 + test1 # These tests work if tol < eps
t2 = 1 + test2
if t2 <= 1:
istop = 2
if t1 <= 1:
istop = 1
if itn >= maxiter:
istop = 6
if Acond >= 0.1/eps:
istop = 4
if epsx >= beta:
istop = 3
# if rnorm <= epsx : istop = 2
# if rnorm <= epsr : istop = 1
if test2 <= tol:
istop = 2
if test1 <= tol:
istop = 1
# See if it is time to print something.
prnt = False
if n <= 40:
prnt = True
if itn <= 10:
prnt = True
if itn >= maxiter-10:
prnt = True
if itn % 10 == 0:
prnt = True
if qrnorm <= 10*epsx:
prnt = True
if qrnorm <= 10*epsr:
prnt = True
if Acond <= 1e-2/eps:
prnt = True
if istop != 0:
prnt = True
if show and prnt:
str1 = '%6g %12.5e %10.3e' % (itn, x[0], test1)
str2 = ' %10.3e' % (test2,)
str3 = ' %8.1e %8.1e %8.1e' % (Anorm, Acond, gbar/Anorm)
print(str1 + str2 + str3)
if itn % 10 == 0:
print()
if callback is not None:
callback(x)
if istop != 0:
break # TODO check this
if show:
print()
print(last + ' istop = %3g itn =%5g' % (istop,itn))
print(last + ' Anorm = %12.4e Acond = %12.4e' % (Anorm,Acond))
print(last + ' rnorm = %12.4e ynorm = %12.4e' % (rnorm,ynorm))
print(last + ' Arnorm = %12.4e' % (Arnorm,))
print(last + msg[istop+1])
if istop == 6:
info = maxiter
else:
info = 0
return (postprocess(x),info)
if __name__ == '__main__':
from scipy import ones, arange
from scipy.linalg import norm
from scipy.sparse import spdiags
n = 10
residuals = []
def cb(x):
residuals.append(norm(b - A*x))
# A = poisson((10,),format='csr')
A = spdiags([arange(1,n+1,dtype=float)], [0], n, n, format='csr')
M = spdiags([1.0/arange(1,n+1,dtype=float)], [0], n, n, format='csr')
A.psolve = M.matvec
b = 0*ones(A.shape[0])
x = minres(A,b,tol=1e-12,maxiter=None,callback=cb)
# x = cg(A,b,x0=b,tol=1e-12,maxiter=None,callback=cb)[0]
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