ℤ 2 -equivariant Ljusternik-Schnirelman theory for non-even functionals
Annales de l'I.H.P. Analyse non linéaire, Tome 15 (1998) no. 3, pp. 341-370. 
@article{AIHPC_1998__15_3_341_0,
     author = {Ekeland, I. and Ghoussoub, N.},
     title = {$\mathbb {Z}_2$-equivariant {Ljusternik-Schnirelman} theory for non-even functionals},
     journal = {Annales de l'I.H.P. Analyse non lin\'eaire},
     pages = {341--370},
     publisher = {Gauthier-Villars},
     volume = {15},
     number = {3},
     year = {1998},
     mrnumber = {1629353},
     zbl = {0907.58006},
     language = {en},
     url = {http://www.numdam.org/item/AIHPC_1998__15_3_341_0/}
}
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Ekeland, I.; Ghoussoub, N. $\mathbb {Z}_2$-equivariant Ljusternik-Schnirelman theory for non-even functionals. Annales de l'I.H.P. Analyse non linéaire, Tome 15 (1998) no. 3, pp. 341-370. http://www.numdam.org/item/AIHPC_1998__15_3_341_0/

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