function[x]=cg_123(A,b,x0)

max_iter=5000;

epsilon=1e-8;

x=x0;

i=0;

r=b-A*x;

d=r;

delta_new=r'*r;

delta0=delta_new;

while(i<max_iter)&&(delta_new/delta0>epsilon^2)

q=A*d;

alpha=delta_new/(d'*q);

x=x+alpha*d;

r=b-A*x;

delta_old=delta_new;

delta_new=r'*r;

beta=delta_new/delta_old;

d=r+beta*d;

i=i+1;

end