function [residu,u]=met_iter(A,M,b,u,epsilon,Nmax)

r=b-A*u;
p=M\r;
k=0;
no=norm(r);
residu=[no];
while no > epsilon & k < Nmax
    u=u+p;
    r=r-A*p;
    p=M\r;
    k=k+1;
    no=norm(r);
    residu=[residu,no];
end