“…Namely, these conditions, combined with the Ekeland principle, make it possible to show that the
set of a studied functionals is nonempty, which leads to the existence of equilibria via Fermat's rule. A similar approach has been applied in case of infinite‐dimensional versions of another milestone result of analysis, that is, the Rolle theorem—see Azagra et al,
4 where the approximate Rolle theorem was established and de B e Silva and Teixeira,
5 where the Palais–Smale condition was employed.…”