Example 4.1-1 Flow near a wall suddenly set in motion
>  restart;
>  pde:=diff(vx(t,y),t)=nu*diff(vx(t,y),y,y);

>  vx:=(t,y)->V*phi(y/sqrt(4*nu*t));

>  pde;
>  de:=simplify(4*t*pde/V);
>  de1:=subs(y=eta*sqrt(4*nu*t),de);

>  s:=dsolve({de1,phi(0)=1,phi(infinity)=0},phi(eta));