clear all

n=5001;
 
A= full(gallery('tridiag',n,-1,2,-1));

x=ones(n,1); b= A*x;  

[xg, FLAG,RELRES,ITER] = pcg(A,b,1e-8,2000);

norm(xg -x, inf)
ITER
norm(A*xg-b,inf)