Definition:Differential/Functional

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Definition

Let $J \sqbrk y$ be a differentiable functional.

Let $h$ be an increment of the independent variable $y$.


Then the term linear with respect to $h$ is called the differential of the functional $J$, and is denoted by $\delta J \sqbrk {y; h}$.


Notes


For a differentiable functional is holds that:

$\Delta J \sqbrk {y;h} = \phi \sqbrk {y; h} + \epsilon \size h$

where $\phi$ is linear with respect to $h$.

Thus:

$\delta J \sqbrk {y; h} = \phi \sqbrk {y; h}$


Also known as

The differential $\delta J \sqbrk {y; h}$ is also known as the (first) variation.


Sources