Question: Collecting similar derivative terms of the variables

Hi,

I need help of your expertize in solving the following queries:

I would start with a sample equation:

Eq1 = u_{xxxx} + u_{xxyy} - v_{xxxy} - v_{xyyy}
where u_{xxxy} represents 4th order pde of u i.e. d^4/(dx^3 dy).

I get a equation similar to Eq1 (but quite complicated) on solving the
equilibrium equations. I would like to collect the similar derivatives
for variable u and v together like

Eq2 = d^2/d x^2 ( u_{xx} + u_{yy} ) - d^2/dx dy ( v_{xx} + v_{yy} )

How to obtain Eq2 from Eq1 ?

Thanks,
Rakesh
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