functional derivative functional/variational derivative functional derivative variational/functional derivative functional/variational derivatives functional derivative functional derivative variational derivative functional/variational derivative functional derivative en.wikipedia.org/wiki/Functional_derivative functional derivative functional/variational derivative functional derivative functional derivative variational/functional derivative variational/functional derivative functional derivative functional derivative functional/variational derivative functional derivative functional derivative functional derivative functional derivative functional/variational functional/variational derivatives functional derivative functional derivative en.wikipedia.org/wiki/… (functional) differentiation functional derivative functional derivative functional/variational derivatives functional/variational derivatives functional derivative functional derivative