Definition:Field of Directions/Functional

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Let $\mathbf y$ be an $N$-dimensional vector.

Let the functional $J$ be such that:

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

Let the following be a family of boundary conditions, presribed $\forall x \in \closedint a b$:

$\mathbf y' = \map {\boldsymbol \psi} {x, \mathbf y}$

Let these boundary conditions be self-adjoint and consistent $\forall x_1, x_2 \in \closedint a b$.

Then these boundary conditions are called field of directions of the functional $J$.