We show the existence
of solutions of the Navier-Stokes equations for which the Dirichlet norm,
∥∇u(t)∥L2(Ω), of the velocity is continuous as t = 0, while the normalized
L2-norm, ∥p(t)∥L2(Ω)∕R, of the pressure is not. This runs counter to the naive
expectation that the relative orders of the spatial derivatives of u, p and ut
should be the same in a priori estimates for the solutions as in the equations
themselves.