qmr (MATLAB Function Reference)

Quasi-Minimal Residual method

Syntax

x = qmr(A,b)
qmr(A,b,tol)
qmr(A,b,tol,maxit)
qmr(A,b,tol,maxit,M)
qmr(A,b,tol,maxit,M1,M2)
qmr(A,b,tol,maxit,M1,M2,x0)
qmr(afun,b,tol,maxit,m1fun,m2fun,x0,p1,p2,...)
[x,flag] = qmr(A,b,...)
[x,flag,relres] = qmr(A,b,...)
[x,flag,relres,iter] = qmr(A,b,...)
[x,flag,relres,iter,resvec] = qmr(A,b,...)

Description

x = qmr(A,b) attempts to solve the system of linear equations A*x=b for x. The n-by-n coefficient matrix A must be square and the column vector b must have length n. A can be a function afun such that afun(x) returns A*x and afun(x,'transp') returns A'*x.

If qmr converges, a message to that effect is displayed. If qmr fails to converge after the maximum number of iterations or halts for any reason, a warning message is printed displaying the relative residual norm(b-A*x)/norm(b) and the iteration number at which the method stopped or failed.

qmr(A,b,tol) specifies the tolerance of the method. If tol is [], then qmr uses the default, 1e-6.

qmr(A,b,tol,maxit) specifies the maximum number of iterations. If maxit is [], then qmr uses the default, min(n,20).

qmr(A,b,tol,maxit,M) and qmr(A,b,tol,maxit,M1,M2) use preconditioners M or M = M1*M2 and effectively solve the system inv(M)*A*x = inv(M)*b for x. If M is [] then qmr applies no preconditioner. M can be a function mfun such that mfun(x) returns M\x and mfun(x,'transp') returns M'\x.

qmr(A,b,tol,maxit,M1,M2,x0) specifies the initial guess. If x0 is [], then qmr uses the default, an all zero vector.

qmr(afun,b,tol,maxit,m1fun,m2fun,x0,p1,p2,...) passes parameters p1,p2,... to functions afun(x,p1,p2,...) and afun(x,p1,p2,...,'transp') and similarly to the preconditioner functions m1fun and m2fun.

[x,flag] = qmr(A,b,...) also returns a convergence flag.

Flag
Convergence

0
qmr converged to the desired tolerance tol within maxit iterations.

1
qmr iterated maxit times but did not converge.

2
Preconditioner M was ill-conditioned.

3
The method stagnated. (Two consecutive iterates were the same.)

4
One of the scalar quantities calculated during qmr became too small or too large to continue computing.

Flag	Convergence
`0`	`qmr` converged to the desired tolerance `tol` within `maxit` iterations.
`1`	`qmr` iterated `maxit` times but did not converge.
`2`	Preconditioner `M` was ill-conditioned.
`3`	The method stagnated. (Two consecutive iterates were the same.)
`4`	One of the scalar quantities calculated during `qmr` became too small or too large to continue computing.

Whenever flag is not 0, the solution x returned is that with minimal norm residual computed over all the iterations. No messages are displayed if the flag output is specified.

[x,flag,relres] = qmr(A,b,...) also returns the relative residual norm(b-A*x)/norm(b). If flag is 0, relres <= tol.

[x,flag,relres,iter] = qmr(A,b,...) also returns the iteration number at which x was computed, where 0 <= iter <= maxit.

[x,flag,relres,iter,resvec] = qmr(A,b,...) also returns a vector of the residual norms at each iteration, including norm(b-A*x0).

Examples

Example 1.

n = 100;
on = ones(n,1);
A = spdiags([-2*on 4*on -on],-1:1,n,n);
b = sum(A,2);
tol = 1e-8; maxit = 15;
M1 = spdiags([on/(-2) on],-1:0,n,n);
M2 = spdiags([4*on -on],0:1,n,n);
x = qmr(A,b,tol,maxit,M1,M2,[]);

Alternatively, use this matrix-vector product function

function y = afun(x,n,transp_flag)
if (nargin > 2) & strcmp(transp_flag,'transp')
   y = 4 * x;
   y(1:n-1) = y(1:n-1) - 2 * x(2:n);
   y(2:n) = y(2:n) - x(1:n-1);
else
   y = 4 * x;
   y(2:n) = y(2:n) - 2 * x(1:n-1);
   y(1:n-1) = y(1:n-1) - x(2:n);
end

as input to qmr

x1 = qmr(@afun,b,tol,maxit,M1,M2,[],n);

Example 2.

load west0479;
A = west0479;
b = sum(A,2);
[x,flag] = qmr(A,b)

flag is 1 because qmr does not converge to the default tolerance 1e-6 within the default 20 iterations.

[L1,U1] = luinc(A,1e-5);
[x1,flag1] = qmr(A,b,1e-6,20,L1,U1)

flag1 is 2 because the upper triangular U1 has a zero on its diagonal, and qmr fails in the first iteration when it tries to solve a system such as U1*y = r for y using backslash.

[L2,U2] = luinc(A,1e-6);
[x2,flag2,relres2,iter2,resvec2] = qmr(A,b,1e-15,10,L2,U2)

flag2 is 0 because qmr converges to the tolerance of 1.6571e-016 (the value of relres2) at the eighth iteration (the value of iter2) when preconditioned by the incomplete LU factorization with a drop tolerance of 1e-6. resvec2(1) = norm(b) and resvec2(9) = norm(b-A*x2). You can follow the progress of qmr by plotting the relative residuals at each iteration starting from the initial estimate (iterate number 0).

semilogy(0:iter2,resvec2/norm(b),'-o')
xlabel('iteration number')
ylabel('relative residual')

See Also

bicg, bicgstab, cgs, gmres, lsqr, luinc, minres, pcg, symmlq

@ (function handle), \ (backslash)

References

[1] Barrett, R., M. Berry, T. F. Chan, et al., Templates for the Solution of Linear Systems: Building Blocks for Iterative Methods, SIAM, Philadelphia, 1994.

[2] Freund, Roland W. and Nöel M. Nachtigal, "QMR: A quasi-minimal residual method for non-Hermitian linear systems", SIAM Journal: Numer. Math. 60, 1991, pp. 315-339.

pwd qr