n=10; l=10; d=0.1; tmax=3; pi=N[Pi]; zuf:=Random[Real,{-1,1}]; u=zuf+Sum[zuf Sin[2 pi i x/l],{i,n}]+ Sum[zuf Cos[2 pi i x/l],{i,n}]; w[t_,x_]=E^t (u/.{a_ Sin[b_]->a Sin[b] Exp[-d (b/x)^2 t], a_ Cos[b_]->a Cos[b] Exp[-d (b/x)^2 t]}); bild=Plot3D[w[t,x],{x,0,l},{t,0,tmax}, AxesLabel->{"x","t","w"},ImageSize->{400,300}];