# Necessary Condition for Integral Functional to have Extremum for given function/Lemma

Jump to navigation
Jump to search

## Theorem

Let $\map \alpha x,\map \beta x$ be real functions.

Let $\map \alpha x,\map \beta x$ be continuous in $\closedint a b$.

Let:

- $\displaystyle\int_a^b \sqbrk{\map \alpha x \map h x+\map \beta x \map {h'} x} \d x = 0 \quad \forall \map h x\in C^1 :\map h a=\map h b=0$,

Then $\map \beta x$ is differentiable.

Furthermore:

- $ \map {\beta'} x=\map \alpha x\quad\forall x\in\closedint a b$.

## Proof

Using integration by parts allows us to factor out $\map h x$:

\(\displaystyle \int_a^b \sqbrk{\map \alpha x \map h x+\map\beta x\map {h'} x }\d x\) | \(=\) | \(\displaystyle \int_a^b \map \alpha x\map h x\d x+\int_a^b \map \beta x\d \map h x\) | where $\d \map h x=\map {h'} x\d x$ | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \displaystyle \int_a^b \map \alpha x \map h x \d x + \map \beta x \map h x \bigg \rvert_a^b - \int_a^b \map h x \d \map \beta x\) | integration by parts | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \displaystyle \int_a^b \sqbrk{\map \alpha x-\map {\beta'} x}\map h x \d x\) | $\map h a=0$, $\map h b=0$ |

Hence, the problem has been reduced to

- $\displaystyle \int_a^b \sqbrk{ \map \alpha x-\map {\beta'} x}\map h x \d x=0$

Since If Definite Integral of a(x)h(x) vanishes for any C^0 h(x) then C^0 a(x) vanishes, the conclusion is that in the considered interval $\closedint a b$ it holds that

- $\map \alpha x=\map {\beta'} x$

$\blacksquare$

## Sources

- 1963: I.M. Gelfand and S.V. Fomin:
*Calculus of Variations*... (previous) ... (next): $\S 1.3$: The Variation of a Functional. A Necessary Condition for an Extremum