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# qmr

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)
[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 should be large and sparse. The column vector b must have length n. You can specify A as a function handle, afun, such that afun(x,'notransp') returns A*x and afun(x,'transp') returns A'*x.

Parameterizing Functions explains how to provide additional parameters to the function afun, as well as the preconditioner function mfun described below, if necessary.

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 handle mfun such that mfun(x,'notransp') 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.

[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.

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

### Using qmr with a Matrix Input

This example shows how to use qmr with a matrix input. The code:

```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);```

displays the message:

```qmr converged at iteration 9 to a solution...
with relative residual
5.6e-009```

### Using qmr with a Function Handle

This example replaces the matrix A in the previous example with a handle to a matrix-vector product function afun. The example is contained in a file run_qmr that

• Calls qmr with the function handle @afun as its first argument.

• Contains afun as a nested function, so that all variables in run_qmr are available to afun.

The following shows the code for run_qmr:

```function x1 = run_qmr
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);
x1 = qmr(@afun,b,tol,maxit,M1,M2);

function y = afun(x,transp_flag)
if strcmp(transp_flag,'transp')      % y = A'*x
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);
elseif strcmp(transp_flag,'notransp') % y = A*x
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
end
end```

When you enter

`x1=run_qmr;`

MATLAB® software displays the message

```qmr converged at iteration 9 to a solution with relative residual
5.6e-009```

### Using a Preconditioner

This example demonstrates the use of a preconditioner.

1. Load A = west0479, a real 479-by-479 nonsymmetric sparse matrix:

```load west0479;
A = west0479;```
2. Define b so that the true solution is a vector of all ones:

```b = full(sum(A,2));
```
3. Set the tolerance and maximum number of iterations:

`tol = 1e-12; maxit = 20;`
4. Use qmr to find a solution at the requested tolerance and number of iterations:

`[x0,fl0,rr0,it0,rv0] = qmr(A,b,tol,maxit);`

fl0 is 1 because qmr does not converge to the requested tolerance 1e-12 within the requested 20 iterations. The seventeenth iterate is the best approximate solution and is the one returned as indicated by it0 =  17. MATLAB stores the residual history in rv0.

5. Plot the behavior of qmr:

```semilogy(0:maxit,rv0/norm(b),'-o');
xlabel('Iteration number');
ylabel('Relative residual');```

The plot shows that the solution does not converge. You can use a preconditioner to improve the outcome.

6. Create the preconditioner with ilu, since the matrix A is nonsymmetric:

```[L,U] = ilu(A,struct('type','ilutp','droptol',1e-5));
Error using ilu
There is a pivot equal to zero. Consider decreasing
the drop tolerance or consider using the 'udiag' option.```

MATLABcannot construct the incomplete LU as it would result in a singular factor, which is useless as a preconditioner.

7. You can try again with a reduced drop tolerance, as indicated by the error message:

```[L,U] = ilu(A,struct('type','ilutp','droptol',1e-6));
[x1,fl1,rr1,it1,rv1] = qmr(A,b,tol,maxit,L,U);
```

fl1 is 0 because qmr drives the relative residual to 4.1410e-014 (the value of rr1). The relative residual is less than the prescribed tolerance of 1e-12 at the sixth iteration (the value of it1) when preconditioned by the incomplete LU factorization with a drop tolerance of 1e-6. The output rv1(1) is norm(b), and the output rv1(7) is norm(b-A*x2).

8. 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:it1,rv1/norm(b),'-o');
xlabel('Iteration number');
ylabel('Relative residual');```

## 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.