It must be called stationary-action principle.
The function $\alpha$ describing a system is the result of a variational problem, the optimization (maximun or minimun) of the functional *action* given by
$$ S(\alpha,t_1,t_2):=\int_{t_1}^{t_2}L(\alpha(t),\dot{\alpha}(t),t)dt $$where $L$ is a function specific for the studied system called the Lagrangian of the system.
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Author of the notes: Antonio J. Pan-Collantes
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