Thismonographdealswiththeexistenceofperiodicmotionsof Lagrangiansystemswith ndegreesoffreedom ij + V'(q) =0, where Visasingularpotential. Aprototypeofsuchaproblem, evenifitisnottheonlyphysicallyinterestingone, istheKepler problem . q 0 q+yqr= . This, jointlywiththemoregeneralN-bodyproblem, hasalways beentheobjectofagreatdealofresearch. Mostofthoseresults arebasedonperturbationmethods, andmakeuseofthespecific featuresoftheKeplerpotential. OurapproachismoreonthelinesofNonlinearFunctional Analysis:ourmainpurposeistogiveafunctionalframefor systemswithsingularpotentials, includingtheKeplerandthe N-bodyproblemasparticularcases. PreciselyweuseCritical PointTheorytoobtainexistenceresults, qualitativeinnature, whichholdtrueforbroadclassesofpotentials. Thishighlights thatthevariationalmethods, whichhavebeenemployedtoob tainimportantadvancesinthestudyofregularHamiltonian systems, canbesuccessfallyusedtohandlesingularpotentials aswell. Theresearchonthistopicisstillinevolution, andtherefore theresultswewillpresentarenottobeintendedasthefinal ones. Indeedamajorpurposeofourdiscussionistopresent methodsandtoolswhichhavebeenusedinstudyingsuchprob lems. Vlll PREFACE Partofthematerialofthisvolumehasbeenpresentedina seriesoflecturesgivenbytheauthorsatSISSA, Trieste, whom wewouldliketothankfortheirhospitalityandsupport. We wishalsotothankUgoBessi, PaoloCaldiroli, FabioGiannoni, LouisJeanjean, LorenzoPisani, EnricoSerra, KazunakaTanaka, EnzoVitillaroforhelpfulsuggestions. May26,1993 Notation n 1. For x, yE IR, x. ydenotestheEuclideanScalarproduct, and IxltheEuclideannorm. 2. meas(A)denotestheLebesguemeasureofthesubset Aof n IR - 3. Wedenoteby ST =[0,T]/{a, T}theunitarycirclepara metrizedby t E[0,T]. Wewillalsowrite SI= ST=I. n 1 n 4. Wewillwrite sn = {xE IR + : Ixl =I}andn = IR \{O}. n 5. Wedenoteby LP([O, T], IR),1~ p~+00,theLebesgue spaces, equippedwiththestandardnorm lIulip. l n l n 6. H (ST, IR)denotestheSobolevspaceof u E H,2(0, T; IR) suchthat u(O) = u(T). Thenormin HIwillbedenoted by lIull2 = lIull~ + lIull~· 7. Wedenoteby(·1·)and11·11respectivelythescalarproduct andthenormoftheHilbertspace E. 8. For uE E, EHilbertorBanachspace, wedenotetheball ofcenter uandradiusrby B(u, r) = {vE E: lIu- vii~ r}. Wewillalsowrite B = B(O, r). r 1 1 9. WesetA (n) = {uE H (St, n)}. k 10. For VE C (1Rxil, IR)wedenoteby V'(t, x)thegradient of Vwithrespectto x. l 11. Given f E C (M, IR), MHilbertmanifold, welet r = {uEM: f(u) ~ a}, f-l(a, b) = {uE E : a~ f(u) ~ b}. x NOTATION 12. Given f E C1(M, JR), MHilbertmanifold, wewilldenote by Zthesetofcriticalpointsof fon Mandby Zctheset Z U f-l(c, c). 13. Givenasequence UnE E, EHilbertspace, by Un --"" Uwe willmeanthatthesequence Unconvergesweaklyto u. 14. With £(E)wewilldenotethesetoflinearandcontinuous operatorson E. 15. With Ck''''(A, JR)wewilldenotethesetoffunctions ffrom AtoJR, ktimesdifferentiablewhosek-derivativeisHolder continuousofexponent0:. Main Assumptions Wecollecthere, forthereader'sconvenience, themainassump tionsonthepotential Vusedthroughoutthebook. (VO) VEC1(lRXO, lR), V(t+T, x)=V(t, X) V(t, x)ElRXO, (VI) V(t, x)

The question of existence and the number of periodic solutions of model equations for a classical mechanical system is a problem as old as the field of analytical mechanics itself. The development of the nonlinear functional analysis has renewed interest in these problems. In this paper we consider a mechanical system which is constrained to a compact manifold M. We suppose that the dynamics of the system is described by a T-periodic Lagrangian L sub t: TM approaches R which satisfies reasonable physical assumptions. The main result of this paper is: If the fundamental group of the manifold M is finite, then the Lagrangian nonlinear system of differential equations which describes the dynamical system has infinitely many distinct periodic solutions. (Author).

This self-contained monograph presents methods for the investigation of nonlinear variational problems. These methods are based on geometric and topological ideas such as topological index, degree of a mapping, Morse-Conley index, Euler characteristics, deformation invariant, homotopic invariant, and the Lusternik-Shnirelman category. Attention is also given to applications in optimisation, mathematical physics, control, and numerical methods. Audience: This volume will be of interest to specialists in functional analysis and its applications, and can also be recommended as a text for graduate and postgraduate-level courses in these fields.

The theory of center manifold reduction is studied in this monograph in the context of (infinite-dimensional) Hamil- tonian and Lagrangian systems. The aim is to establish a "natural reduction method" for Lagrangian systems to their center manifolds. Nonautonomous problems are considered as well assystems invariant under the action of a Lie group ( including the case of relative equilibria). The theory is applied to elliptic variational problemson cylindrical domains. As a result, all bounded solutions bifurcating from a trivial state can be described by a reduced finite-dimensional variational problem of Lagrangian type. This provides a rigorous justification of rod theory from fully nonlinear three-dimensional elasticity. The book will be of interest to researchers working in classical mechanics, dynamical systems, elliptic variational problems, and continuum mechanics. It begins with the elements of Hamiltonian theory and center manifold reduction in order to make the methods accessible to non-specialists, from graduate student level.