function [ z ] = resol(npt,Phi,xi,k,dt )
z=linspace(1,npt,npt)*0;
z(1)=Phi(1);
z(2)=Phi(1)+dt*Phi(2);
for kk=3:npt
    z(kk)=2*z(kk-1)-z(kk-2)+xi*dt*(z(kk-1)-z(kk-2))-dt^2*k*z(kk-1);
end