Now, in this case of irrotational flow, once we have the irrotational flow we have seen what will happen, because we have already seen for a velocity vector q bar. If del u by del x is del u by del y if del v by del x and del v by del v by del; that means, we have del w by del y equal to del v by del z. And then we have del w by del x minus del w by del u by del z this is 0. So, can you find a vector can you find a scalar rather phi which is a function of x y z and t.