(* Group velocity Wavepacket defined by: h[x_,t_]:=Sum[Cos[(i*dk) x-(i*dk)^2*t],{i,1,20}] *********************************************** *) k0=3 dk=0.05 h[x_,t_]:=Sum[Cos[(i*dk) x-(i*dk)^2*t],{i,1,20}] u:=h[x,0] Plot[u,{x,-10,20},PlotRange->{-10,20}] For[ t=0.4, t<4.0,t=t+0.4,u:=h[x,t]; Plot[u,{x,-10,20},PlotRange->{-10,20}]] For[ t=4.0, t<12.0,t=t+0.4,u:=h[x,t]; Plot[u,{x,-10,20},PlotRange->{-10,20}]]