Morse–Palais lemma for nonsmooth functionals on normed spaces
Duong Minh Duc, Tran Vinh Hung, Nguyen Tien Khai · Proceedings of the American Mathematical Society · 2006
Using elementary differential calculus we get a version of the Morse-Palais lemma. Since we do not use powerful tools in functional analysis such as the implicit theorem or flows and deformations in Banach spaces, our result does not require the $C^{1}$-smoothness of functions nor the completeness of spaces. Therefore it is stronger than the classical one but its proof is very simple.