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The Lusternik-Schnirelmann theorem reconsidered. (English) Zbl 0796.57013
Summary: The celebrated theorem of L. Lusternik and L. Schnirelmann [Méthodes topologiques dans les problèmes variationnels (1934; Zbl 0011.02803)], as reformulated by R. S. Palais [Topology 5, 115-132 (1966; Zbl 0143.352)] or J. T. Schwartz [Commun. Pure Appl. Math. 17, 307-315 (1964; Zbl 0152.408)], concerns the behaviour of smooth (i.e., $$C^ 1$$) real-valued functions on a complete Riemannian manifold. The functions are required to be bounded below (or above) and to satisfy one further condition, the Palais-Smale condition. The theorem gives a lower bound, the Lusternik-Schnirelmann category, for the number of critical points of such a function. Without the Palais-Smale condition the result is no longer true. For example the exponential function of a single real variable has no critical points, although it is bounded below and the real line has category one. The purpose of this paper is to establish, by a variant of the classical argument, versions of the Lusternik-Schnirelmann theorem which are valid without the Palais-Smale condition. They reduce to the classical theorem when the manifold is compact and the condition is necessarily satisfied, but in the noncompact case provide new information when the condition is not satisfied.

##### MSC:
 57R70 Critical points and critical submanifolds in differential topology 58E05 Abstract critical point theory (Morse theory, Lyusternik-Shnirel’man theory, etc.) in infinite-dimensional spaces 55M30 Lyusternik-Shnirel’man category of a space, topological complexity à la Farber, topological robotics (topological aspects)
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