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)"
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