## Periodic solutions of Lagrangian systems on a compact manifold.(English)Zbl 0605.58034

The author studies periodic solutions of prescribed period when the Lagrangian system is constrained to a compact manifold. This fact allows him to use many techniques developed in the theory of closed geodesics on Riemannian compact manifolds [cf. the reviewer, Foundations of Global Nonlinear Analysis, Teubner-Texte zur Mathematik, Vol. 86, Leipzig (1986)]. He considers M to be a smooth n-dimensional manifold with TM its tangent bundle. He then considers a time periodic Lagrangian of periodic T, $${\mathcal L}_ t: TM\to R$$, and he searches for T-periodic solutions of the Lagrange equations, which in local coordinates are $(1)\quad \frac{d}{dt}\frac{\partial {\mathcal L}}{\partial \dot q}(t,q,\dot q)- \frac{\partial {\mathcal L}}{\partial q}(t,q,\dot q)=0,\quad i=1,2,...,n.$ The author proves that if the fundamental group of M is finite, then (1) has infinitely many T-periodic solutions, if $${\mathcal L}_ t$$ satisfies certain physically reasonable assumptions. The author makes extensive use of the Palais-Smale condition (C).
Reviewer: T.Rassias

### MSC:

 37G99 Local and nonlocal bifurcation theory for dynamical systems 37J99 Dynamical aspects of finite-dimensional Hamiltonian and Lagrangian systems
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### References:

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