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 $?