P.3.3 Show that the one-dimensional Navier-Stokes equation without pressure gradient (known as the 'viscous' Burger's equation) au +u=- at ax 2x2 is parabolic in x, t. Hint: Write the equation as a system, introducing v=du/ax as second variable and apply equation (3.2.5) to show that the matrix is not of rank 2.