\begin{definition}[Definition:Mutually Consistent Boundary Conditions]
Let $\map {\mathbf y} x$, $\map {\boldsymbol \psi} {\mathbf y}$ be an $n$-dimensional vectors.
Consider the system of differential equations:
:$(1): \quad \mathbf y'' = \map {\mathbf f} {x, \mathbf y, \mathbf y'}$
Let derivatives of $\mathbf y$ satisfy:
:$\bigintlimits {\mathbf y'} {x \mathop = x_1} {} = \bigintlimits {\map {\boldsymbol \psi^{\paren 1} } {\mathbf y} } {x \mathop = x_1} {}$
:$\bigintlimits {\mathbf y'} {x \mathop = x_2} {} = \bigintlimits {\map {\boldsymbol \psi^{\paren 2} } {\mathbf y} } {x \mathop = x_2} {}$
If every solution of $(1)$ satisfying conditions at $x = x_1$ automatically satisfies conditions at $x = x_2$ (or vice versa), then these boundary conditions are called '''mutually consistent'''.
\end{definition}