H-derivative
# H-derivative

In mathematics, the H-derivative is a notion of derivative in the study of abstract Wiener spaces and the Malliavin calculus.

## Definition

Let $i:H\to E$ be an abstract Wiener space, and suppose that $F:E\to \mathbb {R}$ is differentiable. Then the Fréchet derivative is a map

    $\mathrm {D} F:E\to \mathrm {Lin} (E;\mathbb {R} )$;

i.e., for $x\in E$, $\mathrm {D} F(x)$ is an element of $E^{*}$, the dual space to $E$.

Therefore, define the $H$-derivative $\mathrm {D} _{H}F$ at $x\in E$ by

    $\mathrm {D} _{H}F(x):=\mathrm {D} F(x)\circ i:H\to \mathbb {R}$,

a continuous linear map on $H$.

Define the $H$-gradient $\nabla _{H}F:E\to H$ by

    $\langle \nabla _{H}F(x),h\rangle _{H}=\left(\mathrm {D} _{H}F\right)(x)(h)=\lim _{t\to 0}{\frac {F(x+ti(h))-F(x)}{t}}$.

That is, if $j:E^{*}\to H$ denotes the adjoint of $i:H\to E$, we have $\nabla _{H}F(x):=j\left(\mathrm {D} F(x)\right)$.

