# Functional analysis of fa – Higher order functional derivatives

Leave $$E, F$$ Be Banach spaces. A continuous bilinear function $$langle cdot, cdot rangle: E times F to mathbb {R}$$ is named $$E$$-do-degenerate yes $$langle x, y rangle = 0$$ for all $$and in F$$ it implies $$x = 0$$ (Similarly for $$F$$-no-degenerate). Equivalently, the two maps of $$E$$ to $$F *$$ Y $$F$$ to $$E *$$ defined by $$x mapsto langle x, cdot rangle$$ Y $$y mapsto langle cdot, and rangle$$, respectively, are one to one. If they are isomorphisms (*), $$langle cdot, cdot rangle$$ is named $$E$$ or $$F$$-Strongly not degenerate. We say that $$E$$ Y $$F$$ they are in duality if there is a non-degenerated bilinear function $$langle cdot, cdot rangle: E times F to mathbb {R}$$, also called pairing from $$E$$ with $$F$$. If the functional is strongly non-degenerated, we say that the duality is strong.

Consider the following definition.

Definition: Leave $$E$$ Y $$F$$ be regulated spaces and $$langle cdot, cdot rangle$$ a $$E$$– non degenerate matching. Leave $$f: F to mathbb {R}$$ be Fréchet differentiable at the point $$alpha in F$$ (denote this derivative as $$Df (α)$$) The functional derivative $$delta f / delta alpha$$ from $$f$$ with respect to $$alpha$$ It is the unique element in $$E$$, if it exists, so that:
$$begin {eqnarray} Df ( alpha) ( gamma) = langle frac { delta f} { delta alpha}, gamma rangle tag {1} label {1} end {eqnarray}$$
for all $$gamma in F$$.

Now, I would like to know how to define higher order derivatives of functional derivatives. In other words, suppose Fréchet's derivative of $$f$$ to $$alpha$$, $$Df (α)$$ is Fréchet differentiable in $$beta in F$$. Is it possible to define $$delta2 f / delta beta delta alpha$$?