Hi,
I have to compute the variational derivative of some complicated functionals, involving not only derivatives but also finite differences (and other more exotic expressions). Thus I tried
In[1]:= Needs["VariationalMethods`"];
VariationalD[f[x + a]^2, f[x], x]
and got Out[2]= 0, that is incorrect as I expected the answer to be: Out[2]= 2 f[x].
Similarly, with f[x+a]*f[x+b] I get 0 instead of f[x+b-a] + f[x+a-b].
For these simple peculiar examples, I can do what I want with forth-and-back changes of variables. But I have to handle must more complicated functionals that cannot be so easily handled. Hence my question: How can I compute reliably/efficiently such variational derivatives?
Any feedback will be welcome. Thanks. Zep