Formula_25785

\[\frac{d x^{\prime}}{d a}=\xi\left(x^{\prime}, u^{\prime}\right), \quad \frac{d u^{\prime}}{d a}=\eta\left(x^{\prime}, u^{\prime}\right) ;\left.\quad x^{\prime}\right|_{a=0}=x,\left.\quad u^{\prime}\right|_{a=0}=u\]