# Preventing Mathematica from applying product rule

I am programming viscous terms of compressible Navier-Stokes equations

U = {u[t, x, y, z], v[t, x, y, z], w[t, x, y, z]};
a = Div[\[Mu][x, y, z]*(Grad[U, {x, y, z}] + Transpose[Grad[U, {x, y, z}]]), {x, y, z}];


Now, I want to prevent Mathematica from expanding the product rule. In other words I want to have

a = {d/dx( ... ) + d/dy( ... ) + d/dz( ... ),
d/dx( ... ) + d/dy( ... ) + d/dz( ... ),
d/dx( ... ) + d/dy( ... ) + d/dz( ... )}


Can you help me out with that? Thanks in advance!

• use a new name e.g. "ddx" Sep 22 at 16:47

You can use Inactive[Div] instead of Div. When you are ready to evaluate the differentiations, use the substitution /.Inactive[x_]->x.
• You could also use Activate instead of a replacement rule to get rid of the Inactive wrappers Sep 22 at 22:54