In Theorem 10.27 of the book Functional Analysis, Calculus of Variations and Optimal Control, there is the following gradient formula:
($\operatorname{co}$ deotes the convex hull).
Is there an analogue of this result for functions $f:X\to\mathbb R$ on a general Banach (or at least Hilbert) space $X$ and the Fréchet derivative?
Here are the relevant definitions: