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# cgs

## Syntax

x = cgs(A,b)
cgs(A,b,tol)
cgs(A,b,tol,maxit)
cgs(A,b,tol,maxit,M)
cgs(A,b,tol,maxit,M1,M2)
cgs(A,b,tol,maxit,M1,M2,x0)
[x,flag] = cgs(A,b,...)
[x,flag,relres] = cgs(A,b,...)
[x,flag,relres,iter] = cgs(A,b,...)
[x,flag,relres,iter,resvec] = cgs(A,b,...)

## Description

x = cgs(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) 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 cgs converges, a message to that effect is displayed. If cgs 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.

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

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

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

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

[x,flag] = cgs(A,b,...) returns a solution x and a flag that describes the convergence of cgs.

Flag

Convergence

0

cgs converged to the desired tolerance tol within maxititerations.

1

cgs iterated maxit times but did not converge.

2

Preconditioner M was ill-conditioned.

3

cgs stagnated. (Two consecutive iterates were the same.)

4

One of the scalar quantities calculated during cgs 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] = cgs(A,b,...) also returns the relative residual norm(b-A*x)/norm(b). If flag is 0, then relres <= tol.

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

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

## Examples

### Using cgs with a Matrix Input

```A = gallery('wilk',21);
b = sum(A,2);
tol = 1e-12;  maxit = 15;
M1 = diag([10:-1:1 1 1:10]);
x = cgs(A,b,tol,maxit,M1);```

displays the message

```cgs converged at iteration 13 to a solution with
relative residual 2.4e-016.```

### Using cgs with a Function Handle

This example replaces the matrix A in the previous example with a handle to a matrix-vector product function afun, and the preconditioner M1 with a handle to a backsolve function mfun. The example is contained in the file run_cgs that

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

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

The following shows the code for run_cgs:

```function x1 = run_cgs
n = 21;
b = afun(ones(n,1));
tol = 1e-12;  maxit = 15;
x1 = cgs(@afun,b,tol,maxit,@mfun);

function y = afun(x)
y = [0; x(1:n-1)] + ...
[((n-1)/2:-1:0)'; (1:(n-1)/2)'].*x + ...
[x(2:n); 0];
end

function y = mfun(r)
y = r ./ [((n-1)/2:-1:1)'; 1; (1:(n-1)/2)'];
end
end```

When you enter

`x1 = run_cgs`

MATLAB® software returns

```cgs converged at iteration 13 to a solution with
relative residual 2.4e-016.```

### Using a Preconditioner

This example demonstrates the use of a preconditioner.

1. Load 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 cgs to find a solution at the requested tolerance and number of iterations:

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

fl0 is 1 because cgs does not converge to the requested tolerance 1e-12 within the requested 20 iterations. In fact, the behavior of cgs is so poor that the initial guess (x0 = zeros(size(A,2),1)) is the best solution and is returned as indicated by it0 = 0. MATLAB stores the residual history in rv0.

5. Plot the behavior of cgs:

```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 a preconditioner with ilu, since 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.
```

MATLAB cannot 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] = cgs(A,b,tol,maxit,L,U);```

fl1 is 0 because cgs drives the relative residual to 4.3851e-014 (the value of rr1). The relative residual is less than the prescribed tolerance of 1e-12 at the third 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(14) is norm(b-A*x2).

8. You can follow the progress of cgs 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] Sonneveld, Peter, "CGS: A fast Lanczos-type solver for nonsymmetric linear systems," SIAM J. Sci. Stat. Comput., January 1989, Vol. 10, No. 1, pp. 36–52.