I have the following
$$ \frac{d}{dn(x)} \int_{x \in \cal{R}^3} {n(x) dx} $$
I know that this additional relationship holds
$$ \int_{x \in \cal{R}^3}{n(x) dx} = N $$
where N is a constant.
My question is, what is the value of the above derivative, and what is the procedure for this case? I am not really experienced in functionals and functional derivatives.