MATLAB Function Reference Volume 1: A - E - Bad Request

icg
2-148
maxit = 15;
M1 = spdiags([on/(-2) on],-1:0,n,n);
M2 = spdiags([4*on -on],0:1,n,n);
x = bicg(A,b,tol,maxit,M1,M2,[]);
bicg converged at iteration 9 to a solution with relative
residual 5.3e-009
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 bicg.
x1 = bicg(@afun,b,tol,maxit,M1,M2,[],n);
Example 2. Start with A = west0479 and make the true solution the vector of
all ones.
load west0479;
A = west0479;
b = sum(A,2);
You can accurately solve A*x = b using backslash since A is not so large.
x = A \ b;
norm(b-A*x) / norm(b)
ans =
1.2454e-017
Now try to solve A*x = b with bicg.
[x,flag,relres,iter,resvec] = bicg(A,b)