Definition:Conjugate Point (Calculus of Variations)

Definition 1
Let


 * $-\map {\dfrac \d {\d x} } {P h'} + Q h = 0$

with boundary conditions


 * $\map h a = 0, \quad \map h c = 0, \quad a < c \le b$

Suppose


 * $\map h x = 0 \quad \neg \forall x \in \closedint a b$

Suppose


 * $\map h a = 0, \quad \map h {\tilde a} = 0, \quad a \ne \tilde a$

Then the point $\tilde a$ is called conjugate to the point $a$ solution to the aforementioned differential equation.

Definition 2
Let $y = \map y x$ and $y^* = \map {y^*} x$ be extremal functions.

Let


 * $M = \paren {a, \map y a}$


 * $\tilde M = \paren {\tilde a, \map y {\tilde a} }$

Let $y$ and $y^*$ both pass through the point $M$.

Let


 * $\map {y^*} {x - \tilde a} - \map y {x - \tilde a} = \epsilon \size {\map {y^*} {x - \tilde a} - \map y {x - \tilde a} }_1$

where:


 * $\size {\map {y^*} {x - \tilde a} - \map y {x - \tilde a} }_1 \to 0 \implies \epsilon \to 0$

Then $\tilde M$ is conjugate to $M$.

Definition 3
Let $y = \map y x$ and $y = \map {\tilde y} x$ be extremal functions.

Let


 * $M = \paren {a, \map y a}$


 * $\tilde M = \paren {\tilde a, \map y {\tilde a} }$

Let both $y = \map y x$ and $y = \map {\tilde y} x$ pass through the point $M$.

Let


 * $\displaystyle \lim_{\size {\map y x - \map {\tilde y} x}_1 \to 0} \sqbrk {\paren {x, \map y x}: \map y x - \map {\tilde y} x = 0} = \tilde M$

In other words, let $\tilde M$ be the limit of points of intersection of $y = \map y x$ and $y = \map {\tilde y} x$ as $\size {\map y x - \map {\tilde y} x}_1 \to 0$

Then $\tilde M$ is conjugate to $M$.

Also defined as
In the context of Calculus of Variations, functionals are one of the most important concepts.

Therefore, instead of a function, a functional which is minimised by the given function is used as a concept of reference.

Then, if $\tilde a$ is conjugate to $a$ solution of $\sqbrk {-\map {\dfrac \d {\d x} } {P h'} + Q h = 0}$, then it is also conjugate  $\displaystyle \int_a^b \paren {P h'^2 + Q h^2} \rd x$.