Legendre's Condition/Lemma 1

Lemma
Let $y = \map y x$ be a real function, such that:


 * $\map y a = A$
 * $\map y b = B$

Let $J \sqbrk y$ be a functional, such that:


 * $\displaystyle J \sqbrk y = \int_a^b \map F {x, y, y'} \rd x$

where:


 * $F \in C^2 \closedint a b$

all its variables.

Then:


 * $\displaystyle \delta^2 J \sqbrk {y; h} = \int_a^b \paren {\map P {x, \map y x} h'^2 + \map Q {x, \map y x} h^2} \rd x$

where:

Proof
The minimising function $y$ has fixed end-points.

Therefore, consider an increment of a functional with $h$ such that:


 * $h \in C^1 \closedint a b: \paren {\map h a = 0} \land \paren {\map h b = 0}$

Then:

where omitted variables are $\paren {x, y, y'}$, and the overbar indicates derivatives being taken along some intermediate curves:

with $0 < \theta < 1$.

If $\overline F_{yy}$, $\overline F_{yy'}$, $\overline F_{y'y'} $ are to be replaced by $F_{yy}$, $F_{yy}$, $F_{y'y'}$ evaluated at the point $\tuple {x, \map y x, \map {y'} x}$, then:


 * $\displaystyle \Delta J \sqbrk {y; h} = \int_a^b\left[{\map {F_y} {x, y, y'} h + \map {F_{y'} } {x, y, y'} h'}\right] \rd x + \frac 1 2 \int_a^b \sqbrk {\map {F_{yy} } {x, y, y'} h^2 + 2 \map {F_{yy'} } {x, y, y'} h h'+ \map {F_{y'y'} } {x, y, y'} h'^2} \rd x + \epsilon$

where:


 * $\displaystyle \epsilon = \int_a^b \sqbrk {\epsilon_1 h^2 + \epsilon_2 h h' + \epsilon_3 h'^2}$

By continuity of $F_{yy}$, $F_{yy}$, $F_{y'y'}$:


 * $\size h_1 \to 0 \implies \epsilon_1, \epsilon_2, \epsilon_3 \to 0$

Thus, $\epsilon$ is an infinitesimal of the order higher than 2 $\size h$.

The first and second term on the of $\Delta J \sqbrk {y; h}$ are $\delta J \sqbrk {y; h}$ and $\delta^2 J \sqbrk {y; h}$ respectively.

Integrate the second term of $\delta^2 J \sqbrk {y; h}$ by parts:

Therefore:


 * $\displaystyle \delta^2 J \sqbrk {y; h} = \int_a^b \paren {\frac 1 2 F_{y'y'} h'^2 + \frac 1 2 \sqbrk {F_{yy} - \frac \d {\d x} F_{yy'} } h^2} \rd x$

Mistake

 * $\S 5.25$: The Formula for the Second Variation. Legendre's Condition p. 102

states that


 * $P = \dfrac 1 2 F_{y'y'} \quad Q = \dfrac 1 2 \paren {F_{yy'} - \dfrac \d {\d x} F_{yy'} }$

This is a mistake, since the second variation should contain both pure and mixed partial derivatives of the order 2.

However, $F_{yy} $ is missing and could not have been lost during derivation of the proof.

It should be:


 * $Q = \dfrac 1 2 \paren {F_{yy} - \dfrac \d {\d x} F_{yy'} }$