IoTa-pub

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\begin{document}

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\frontmatter % for the preliminaries

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\pagestyle{headings} % switches on printing of running heads

\addtocmark{Hamiltonian Mechanics} % additional mark in the TOC

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\chapter*{Preface}

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This textbook is intended for use by students of physics, physical

chemistry, and theoretical chemistry. The reader is presumed to have a

basic knowledge of atomic and quantum physics at the level provided, for

example, by the first few chapters in our book {\it The Physics of Atoms

and Quanta}. The student of physics will find here material which should

be included in the basic education of every physicist. This book should

furthermore allow students to acquire an appreciation of the breadth and

variety within the field of molecular physics and its future as a

fascinating area of research.

For the student of chemistry, the concepts introduced in this book will

provide a theoretical framework for that entire field of study. With the

help of these concepts, it is at least in principle possible to reduce

the enormous body of empirical chemical knowledge to a few basic

principles: those of quantum mechanics. In addition, modern physical

methods whose fundamentals are introduced here are becoming increasingly

important in chemistry and now represent indispensable tools for the

chemist. As examples, we might mention the structural analysis of

complex organic compounds, spectroscopic investigation of very rapid

reaction processes or, as a practical application, the remote detection

of pollutants in the air.

\vspace{1cm}

\begin{flushright}\noindent

April 1995\hfill Walter Olthoff\\

Program Chair\\

ECOOP'95

\end{flushright}

%

\chapter*{Organization}

ECOOP'95 is organized by the department of Computer Science, Univeristy

of \AA rhus and AITO (association Internationa pour les Technologie

Object) in cooperation with ACM/SIGPLAN.

%

\section*{Executive Commitee}

\begin{tabular}{@{}p{5cm}@{}p{7.2cm}@{}}

Conference Chair:&Ole Lehrmann Madsen (\AA rhus University, DK)\\

Program Chair: &Walter Olthoff (DFKI GmbH, Germany)\\

Organizing Chair:&J\o rgen Lindskov Knudsen (\AA rhus University, DK)\\

Tutorials:&Birger M\o ller-Pedersen\hfil\break

(Norwegian Computing Center, Norway)\\

Workshops:&Eric Jul (University of Kopenhagen, Denmark)\\

Panels:&Boris Magnusson (Lund University, Sweden)\\

Exhibition:&Elmer Sandvad (\AA rhus University, DK)\\

Demonstrations:&Kurt N\o rdmark (\AA rhus University, DK)

\end{tabular}

%

\section*{Program Commitee}

\begin{tabular}{@{}p{5cm}@{}p{7.2cm}@{}}

Conference Chair:&Ole Lehrmann Madsen (\AA rhus University, DK)\\

Program Chair: &Walter Olthoff (DFKI GmbH, Germany)\\

Organizing Chair:&J\o rgen Lindskov Knudsen (\AA rhus University, DK)\\

Tutorials:&Birger M\o ller-Pedersen\hfil\break

(Norwegian Computing Center, Norway)\\

Workshops:&Eric Jul (University of Kopenhagen, Denmark)\\

Panels:&Boris Magnusson (Lund University, Sweden)\\

Exhibition:&Elmer Sandvad (\AA rhus University, DK)\\

Demonstrations:&Kurt N\o rdmark (\AA rhus University, DK)

\end{tabular}

%

\begin{multicols}{3}[\section*{Referees}]

V.~Andreev\\

B\"arwolff\\

E.~Barrelet\\

H.P.~Beck\\

G.~Bernardi\\

E.~Binder\\

P.C.~Bosetti\\

Braunschweig\\

F.W.~B\"usser\\

T.~Carli\\

A.B.~Clegg\\

G.~Cozzika\\

S.~Dagoret\\

Del~Buono\\

P.~Dingus\\

H.~Duhm\\

J.~Ebert\\

S.~Eichenberger\\

R.J.~Ellison\\

Feltesse\\

W.~Flauger\\

A.~Fomenko\\

G.~Franke\\

J.~Garvey\\

M.~Gennis\\

L.~Goerlich\\

P.~Goritchev\\

H.~Greif\\

E.M.~Hanlon\\

R.~Haydar\\

R.C.W.~Henderso\\

P.~Hill\\

H.~Hufnagel\\

A.~Jacholkowska\\

Johannsen\\

S.~Kasarian\\

I.R.~Kenyon\\

C.~Kleinwort\\

T.~K\"ohler\\

S.D.~Kolya\\

P.~Kostka\\

U.~Kr\"uger\\

J.~Kurzh\"ofer\\

M.P.J.~Landon\\

A.~Lebedev\\

Ch.~Ley\\

F.~Linsel\\

H.~Lohmand\\

Martin\\

S.~Masson\\

K.~Meier\\

C.A.~Meyer\\

S.~Mikocki\\

J.V.~Morris\\

B.~Naroska\\

Nguyen\\

U.~Obrock\\

G.D.~Patel\\

Ch.~Pichler\\

S.~Prell\\

F.~Raupach\\

V.~Riech\\

P.~Robmann\\

N.~Sahlmann\\

P.~Schleper\\

Sch\"oning\\

B.~Schwab\\

A.~Semenov\\

G.~Siegmon\\

J.R.~Smith\\

M.~Steenbock\\

U.~Straumann\\

C.~Thiebaux\\

P.~Van~Esch\\

from Yerevan Ph\\

L.R.~West\\

G.-G.~Winter\\

T.P.~Yiou\\

M.~Zimmer\end{multicols}

%

\section*{Sponsoring Institutions}

%

Bernauer-Budiman Inc., Reading, Mass.\\

The Hofmann-International Company, San Louis Obispo, Cal.\\

Kramer Industries, Heidelberg, Germany

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\tableofcontents

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\mainmatter % start of the contributions

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\title{Hamiltonian Mechanics unter besonderer Ber\"ucksichtigung der

h\"ohreren Lehranstalten}

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\titlerunning{Hamiltonian Mechanics} % abbreviated title (for running head)

% also used for the TOC unless

% \toctitle is used

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\author{Ivar Ekeland\inst{1} \and Roger Temam\inst{2}

Jeffrey Dean \and David Grove \and Craig Chambers \and Kim~B.~Bruce \and

Elsa Bertino}

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\authorrunning{Ivar Ekeland et al.} % abbreviated author list (for running head)

%

%%%% modified list of authors for the TOC (add the affiliations)

\tocauthor{Ivar Ekeland (Princeton University),

Roger Temam (Universit\'{e} de Paris-Sud),

Jeffrey Dean, David Grove, Craig Chambers (Universit\`a di Geova),

Kim B. Bruce (Stanford University),

Elisa Bertino (Digita Research Center)}

%

\institute{Princeton University, Princeton NJ 08544, USA,\\

\email{I.Ekeland@princeton.edu},\\ WWW home page:

\texttt{http://users/\homedir iekeland/web/welcome.html}

\and

Universit\'{e} de Paris-Sud,

Laboratoire d'Analyse Num\'{e}rique, B\^{a}timent 425,\\

F-91405 Orsay Cedex, France}

\maketitle % typeset the title of the contribution

\begin{abstract}

The abstract should summarize the contents of the paper

using at least 70 and at most 150 words. It will be set in 9-point

font size and be inset 1.0 cm from the right and left margins.

There will be two blank lines before and after the Abstract. \dots

\end{abstract}

%

\section{Fixed-Period Problems: The Sublinear Case}

%

With this chapter, the preliminaries are over, and we begin the search

for periodic solutions to Hamiltonian systems. All this will be done in

the convex case; that is, we shall study the boundary-value problem

\begin{eqnarray*}

\dot{x}&=&JH' (t,x)\\

x(0) &=& x(T)

\end{eqnarray*}

with $H(t,\cdot)$ a convex function of $x$, going to $+\infty$ when

$\left\|x\right\| \to \infty$.

%

\subsection{Autonomous Systems}

%

In this section, we will consider the case when the Hamiltonian $H(x)$

is autonomous. For the sake of simplicity, we shall also assume that it

is $C^{1}$.

We shall first consider the question of nontriviality, within the

general framework of

$\left(A_{\infty},B_{\infty}\right)$-subquadratic Hamiltonians. In

the second subsection, we shall look into the special case when $H$ is

$\left(0,b_{\infty}\right)$-subquadratic,

and we shall try to derive additional information.

%

\subsubsection{The General Case: Nontriviality.}

%

We assume that $H$ is

$\left(A_{\infty},B_{\infty}\right)$-sub\-qua\-dra\-tic at infinity,

for some constant symmetric matrices $A_{\infty}$ and $B_{\infty}$,

with $B_{\infty}-A_{\infty}$ positive definite. Set:

\begin{eqnarray}

\gamma :&=&{\rm smallest\ eigenvalue\ of}\ \ B_{\infty} - A_{\infty} \\

\lambda : &=& {\rm largest\ negative\ eigenvalue\ of}\ \

J \frac{d}{dt} +A_{\infty}\ .

\end{eqnarray}

Theorem~\ref{ghou:pre} tells us that if $\lambda +\gamma < 0$, the

boundary-value problem:

\begin{equation}

\begin{array}{rcl}

\dot{x}&=&JH' (x)\\

x(0)&=&x (T)

\end{array}

\end{equation}

has at least one solution

$\overline{x}$, which is found by minimizing the dual

action functional:

\begin{equation}

\psi (u) = \int_{o}^{T} \left[\frac{1}{2}

\left(\Lambda_{o}^{-1} u,u\right) + N^{\ast} (-u)\right] dt

\end{equation}

on the range of $\Lambda$, which is a subspace $R (\Lambda)_{L}^{2}$

with finite codimension. Here

\begin{equation}

N(x) := H(x) - \frac{1}{2} \left(A_{\infty} x,x\right)

\end{equation}

is a convex function, and

\begin{equation}

N(x) \le \frac{1}{2}

\left(\left(B_{\infty} - A_{\infty}\right) x,x\right)

+ c\ \ \ \forall x\ .

\end{equation}

%

\begin{proposition}

Assume $H'(0)=0$ and $ H(0)=0$. Set:

\begin{equation}

\delta := \liminf_{x\to 0} 2 N (x) \left\|x\right\|^{-2}\ .

\label{eq:one}

\end{equation}

If $\gamma < - \lambda < \delta$,

the solution $\overline{u}$ is non-zero:

\begin{equation}

\overline{x} (t) \ne 0\ \ \ \forall t\ .

\end{equation}

\end{proposition}

%

\begin{proof}

Condition (\ref{eq:one}) means that, for every

$\delta ' > \delta$, there is some $\varepsilon > 0$ such that

\begin{equation}

\left\|x\right\| \le \varepsilon \Rightarrow N (x) \le

\frac{\delta '}{2} \left\|x\right\|^{2}\ .

\end{equation}

It is an exercise in convex analysis, into which we shall not go, to

show that this implies that there is an $\eta > 0$ such that

\begin{equation}

f\left\|x\right\| \le \eta

\Rightarrow N^{\ast} (y) \le \frac{1}{2\delta '}

\left\|y\right\|^{2}\ .

\label{eq:two}

\end{equation}

\begin{figure}

\vspace{2.5cm}

\caption{This is the caption of the figure displaying a white eagle and

a white horse on a snow field}

\end{figure}

Since $u_{1}$ is a smooth function, we will have

$\left\|hu_{1}\right\|_\infty \le \eta$

for $h$ small enough, and inequality (\ref{eq:two}) will hold,

yielding thereby:

\begin{equation}

\psi (hu_{1}) \le \frac{h^{2}}{2}

\frac{1}{\lambda} \left\|u_{1} \right\|_{2}^{2} + \frac{h^{2}}{2}

\frac{1}{\delta '} \left\|u_{1}\right\|^{2}\ .

\end{equation}

If we choose $\delta '$ close enough to $\delta$, the quantity

$\left(\frac{1}{\lambda} + \frac{1}{\delta '}\right)$

will be negative, and we end up with

\begin{equation}

\psi (hu_{1}) < 0\ \ \ \ \ {\rm for}\ \ h\ne 0\ \ {\rm small}\ .

\end{equation}

On the other hand, we check directly that $\psi (0) = 0$. This shows

that 0 cannot be a minimizer of $\psi$, not even a local one.

So $\overline{u} \ne 0$ and

$\overline{u} \ne \Lambda_{o}^{-1} (0) = 0$. \qed

\end{proof}

%

\begin{corollary}

Assume $H$ is $C^{2}$ and

$\left(a_{\infty},b_{\infty}\right)$-subquadratic at infinity. Let

$\xi_{1},\allowbreak\dots,\allowbreak\xi_{N}$ be the

equilibria, that is, the solutions of $H' (\xi ) = 0$.

Denote by $\omega_{k}$

the smallest eigenvalue of $H'' \left(\xi_{k}\right)$, and set:

\begin{equation}

\omega : = {\rm Min\,} \left\{\omega_{1},\dots,\omega_{k}\right\}\ .

\end{equation}

If:

\begin{equation}

\frac{T}{2\pi} b_{\infty} <

- E \left[- \frac{T}{2\pi}a_{\infty}\right] <

\frac{T}{2\pi}\omega

\label{eq:three}

\end{equation}

then minimization of $\psi$ yields a non-constant $T$-periodic solution

$\overline{x}$.

\end{corollary}

%

We recall once more that by the integer part $E [\alpha ]$ of

$\alpha \in \bbbr$, we mean the $a\in \bbbz$

such that $a< \alpha \le a+1$. For instance,

if we take $a_{\infty} = 0$, Corollary 2 tells

us that $\overline{x}$ exists and is

non-constant provided that:

\begin{equation}

\frac{T}{2\pi} b_{\infty} < 1 < \frac{T}{2\pi}

\end{equation}

or

\begin{equation}

T\in \left(\frac{2\pi}{\omega},\frac{2\pi}{b_{\infty}}\right)\ .

\label{eq:four}

\end{equation}

%

\begin{proof}

The spectrum of $\Lambda$ is $\frac{2\pi}{T} \bbbz +a_{\infty}$. The

largest negative eigenvalue $\lambda$ is given by

$\frac{2\pi}{T}k_{o} +a_{\infty}$,

where

\begin{equation}

\frac{2\pi}{T}k_{o} + a_{\infty} < 0

\le \frac{2\pi}{T} (k_{o} +1) + a_{\infty}\ .

\end{equation}

Hence:

\begin{equation}

k_{o} = E \left[- \frac{T}{2\pi} a_{\infty}\right] \ .

\end{equation}

The condition $\gamma < -\lambda < \delta$ now becomes:

\begin{equation}

b_{\infty} - a_{\infty} <

- \frac{2\pi}{T} k_{o} -a_{\infty} < \omega -a_{\infty}

\end{equation}

which is precisely condition (\ref{eq:three}).\qed

\end{proof}

%

\begin{lemma}

Assume that $H$ is $C^{2}$ on $\bbbr^{2n} \setminus \{ 0\}$ and

that $H'' (x)$ is non-de\-gen\-er\-ate for any $x\ne 0$. Then any local

minimizer $\widetilde{x}$ of $\psi$ has minimal period $T$.

\end{lemma}

%

\begin{proof}

We know that $\widetilde{x}$, or

$\widetilde{x} + \xi$ for some constant $\xi

\in \bbbr^{2n}$, is a $T$-periodic solution of the Hamiltonian system:

\begin{equation}

\dot{x} = JH' (x)\ .

\end{equation}

There is no loss of generality in taking $\xi = 0$. So

$\psi (x) \ge \psi (\widetilde{x} )$

for all $\widetilde{x}$ in some neighbourhood of $x$ in

$W^{1,2} \left(\bbbr / T\bbbz ; \bbbr^{2n}\right)$.

But this index is precisely the index

$i_{T} (\widetilde{x} )$ of the $T$-periodic

solution $\widetilde{x}$ over the interval

$(0,T)$, as defined in Sect.~2.6. So

\begin{equation}

i_{T} (\widetilde{x} ) = 0\ .

\label{eq:five}

\end{equation}

Now if $\widetilde{x}$ has a lower period, $T/k$ say,

we would have, by Corollary 31:

\begin{equation}

i_{T} (\widetilde{x} ) =

i_{kT/k}(\widetilde{x} ) \ge

ki_{T/k} (\widetilde{x} ) + k-1 \ge k-1 \ge 1\ .

\end{equation}

This would contradict (\ref{eq:five}), and thus cannot happen.\qed

\end{proof}

%

\paragraph{Notes and Comments.}

The results in this section are a

refined version of \cite{clar:eke};

the minimality result of Proposition

14 was the first of its kind.

To understand the nontriviality conditions, such as the one in formula

(\ref{eq:four}), one may think of a one-parameter family

$x_{T}$, $T\in \left(2\pi\omega^{-1}, 2\pi b_{\infty}^{-1}\right)$

of periodic solutions, $x_{T} (0) = x_{T} (T)$,

with $x_{T}$ going away to infinity when $T\to 2\pi \omega^{-1}$,

which is the period of the linearized system at 0.

\begin{table}

\caption{This is the example table taken out of {\it The

\TeX{}book,} p.\,246}

\begin{center}

\begin{tabular}{r@{\quad}rl}

\hline

\multicolumn{1}{l}{\rule{0pt}{12pt}

Year}&\multicolumn{2}{l}{World population}\\[2pt]

\hline\rule{0pt}{12pt}

8000 B.C. & 5,000,000& \\

50 A.D. & 200,000,000& \\

1650 A.D. & 500,000,000& \\

1945 A.D. & 2,300,000,000& \\

1980 A.D. & 4,400,000,000& \\[2pt]

\hline

\end{tabular}

\end{center}

\end{table}

%

\begin{theorem} [Ghoussoub-Preiss]\label{ghou:pre}

Assume $H(t,x)$ is

$(0,\varepsilon )$-subquadratic at

infinity for all $\varepsilon > 0$, and $T$-periodic in $t$

\begin{equation}

H (t,\cdot )\ \ \ \ \ {\rm is\ convex}\ \ \forall t

\end{equation}

\begin{equation}

H (\cdot ,x)\ \ \ \ \ {\rm is}\ \ T{\rm -periodic}\ \ \forall x

\end{equation}

\begin{equation}

H (t,x)\ge n\left(\left\|x\right\|\right)\ \ \ \ \

{\rm with}\ \ n (s)s^{-1}\to \infty\ \ {\rm as}\ \ s\to \infty

\end{equation}

\begin{equation}

\forall \varepsilon > 0\ ,\ \ \ \exists c\ :\

H(t,x) \le \frac{\varepsilon}{2}\left\|x\right\|^{2} + c\ .

\end{equation}

Assume also that $H$ is $C^{2}$, and $H'' (t,x)$ is positive definite

everywhere. Then there is a sequence $x_{k}$, $k\in \bbbn$, of

$kT$-periodic solutions of the system

\begin{equation}

\dot{x} = JH' (t,x)

\end{equation}

such that, for every $k\in \bbbn$, there is some $p_{o}\in\bbbn$ with:

\begin{equation}

p\ge p_{o}\Rightarrow x_{pk} \ne x_{k}\ .

\end{equation}

\qed

\end{theorem}

%

\begin{example} [{{\rm External forcing}}]

Consider the system:

\begin{equation}

\dot{x} = JH' (x) + f(t)

\end{equation}

where the Hamiltonian $H$ is

$\left(0,b_{\infty}\right)$-subquadratic, and the

forcing term is a distribution on the circle:

\begin{equation}

f = \frac{d}{dt} F + f_{o}\ \ \ \ \

{\rm with}\ \ F\in L^{2} \left(\bbbr / T\bbbz; \bbbr^{2n}\right)\ ,

\end{equation}

where $f_{o} : = T^{-1}\int_{o}^{T} f (t) dt$. For instance,

\begin{equation}

f (t) = \sum_{k\in \bbbn} \delta_{k} \xi\ ,

\end{equation}

where $\delta_{k}$ is the Dirac mass at $t= k$ and

$\xi \in \bbbr^{2n}$ is a

constant, fits the prescription. This means that the system

$\dot{x} = JH' (x)$ is being excited by a

series of identical shocks at interval $T$.

\end{example}

%

\begin{definition}

Let $A_{\infty} (t)$ and $B_{\infty} (t)$ be symmetric

operators in $\bbbr^{2n}$, depending continuously on

$t\in [0,T]$, such that

$A_{\infty} (t) \le B_{\infty} (t)$ for all $t$.

A Borelian function

$H: [0,T]\times \bbbr^{2n} \to \bbbr$

is called

$\left(A_{\infty} ,B_{\infty}\right)$-{\it subquadratic at infinity}

if there exists a function $N(t,x)$ such that:

\begin{equation}

H (t,x) = \frac{1}{2} \left(A_{\infty} (t) x,x\right) + N(t,x)

\end{equation}

\begin{equation}

\forall t\ ,\ \ \ N(t,x)\ \ \ \ \

{\rm is\ convex\ with\ respect\ to}\ \ x

\end{equation}

\begin{equation}

N(t,x) \ge n\left(\left\|x\right\|\right)\ \ \ \ \

{\rm with}\ \ n(s)s^{-1}\to +\infty\ \ {\rm as}\ \ s\to +\infty

\end{equation}

\begin{equation}

\exists c\in \bbbr\ :\ \ \ H (t,x) \le

\frac{1}{2} \left(B_{\infty} (t) x,x\right) + c\ \ \ \forall x\ .

\end{equation}

If $A_{\infty} (t) = a_{\infty} I$ and

$B_{\infty} (t) = b_{\infty} I$, with

$a_{\infty} \le b_{\infty} \in \bbbr$,

we shall say that $H$ is

$\left(a_{\infty},b_{\infty}\right)$-subquadratic

at infinity. As an example, the function

$\left\|x\right\|^{\alpha}$, with

$1\le \alpha < 2$, is $(0,\varepsilon )$-subquadratic at infinity

for every $\varepsilon > 0$. Similarly, the Hamiltonian

\begin{equation}

H (t,x) = \frac{1}{2} k \left\|k\right\|^{2} +\left\|x\right\|^{\alpha}

\end{equation}

is $(k,k+\varepsilon )$-subquadratic for every $\varepsilon > 0$.

Note that, if $k<0$, it is not convex.

\end{definition}

%

\paragraph{Notes and Comments.}

The first results on subharmonics were

obtained by Rabinowitz in \cite{rab}, who showed the existence of

infinitely many subharmonics both in the subquadratic and superquadratic

case, with suitable growth conditions on $H'$. Again the duality

approach enabled Clarke and Ekeland in \cite{clar:eke:2} to treat the

same problem in the convex-subquadratic case, with growth conditions on

$H$ only.

Recently, Michalek and Tarantello (see \cite{mich:tar} and \cite{tar})

have obtained lower bound on the number of subharmonics of period $kT$,

based on symmetry considerations and on pinching estimates, as in

Sect.~5.2 of this article.

%

% ---- Bibliography ----

%

\begin{thebibliography}{5}

%

\bibitem {clar:eke}

Clarke, F., Ekeland, I.:

Nonlinear oscillations and

boundary-value problems for Hamiltonian systems.

Arch. Rat. Mech. Anal. {\bf 78} (1982) 315--333

\bibitem {clar:eke:2}

Clarke, F., Ekeland, I.:

Solutions p\'{e}riodiques, du

p\'{e}riode donn\'{e}e, des \'{e}quations hamiltoniennes.

Note CRAS Paris {\bf 287} (1978) 1013--1015

\bibitem {mich:tar}

Michalek, R., Tarantello, G.:

Subharmonic solutions with prescribed minimal

period for nonautonomous Hamiltonian systems.

J. Diff. Eq. {\bf 72} (1988) 28--55

\bibitem {tar}

Tarantello, G.:

Subharmonic solutions for Hamiltonian

systems via a $\bbbz_{p}$ pseudoindex theory.

Annali di Matematica Pura (to appear)

\bibitem {rab}

Rabinowitz, P.:

On subharmonic solutions of a Hamiltonian system.

Comm. Pure Appl. Math. {\bf 33} (1980) 609--633

\end{thebibliography}

%

% second contribution with nearly identical text,

% slightly changed contribution head (all entries

% appear as defaults), and modified bibliography

%

\title{Hamiltonian Mechanics2}

\author{Ivar Ekeland\inst{1} \and Roger Temam\inst{2}}

\institute{Princeton University, Princeton NJ 08544, USA

\and

Universit\'{e} de Paris-Sud,

Laboratoire d'Analyse Num\'{e}rique, B\^{a}timent 425,\\

F-91405 Orsay Cedex, France}

\maketitle

%

% Modify the bibliography environment to call for the author-year

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\makeatletter

\renewenvironment{thebibliography}[1]

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\def\@cite#1{#1}%

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{\def\protect##1{\string ##1\space}\immediate

\write\@auxout{\string\bibcite{#2}{#1}}}\fi\ignorespaces}

\makeatother

%

\begin{abstract}

The abstract should summarize the contents of the paper

using at least 70 and at most 150 words. It will be set in 9-point

font size and be inset 1.0 cm from the right and left margins.

There will be two blank lines before and after the Abstract. \dots

\end{abstract}

%

\section{Fixed-Period Problems: The Sublinear Case}

%

With this chapter, the preliminaries are over, and we begin the search

for periodic solutions to Hamiltonian systems. All this will be done in

the convex case; that is, we shall study the boundary-value problem

\begin{eqnarray*}

\dot{x}&=&JH' (t,x)\\

x(0) &=& x(T)

\end{eqnarray*}

with $H(t,\cdot)$ a convex function of $x$, going to $+\infty$ when

$\left\|x\right\| \to \infty$.

%

\subsection{Autonomous Systems}

%

In this section, we will consider the case when the Hamiltonian $H(x)$

is autonomous. For the sake of simplicity, we shall also assume that it

is $C^{1}$.

We shall first consider the question of nontriviality, within the

general framework of

$\left(A_{\infty},B_{\infty}\right)$-subquadratic Hamiltonians. In

the second subsection, we shall look into the special case when $H$ is

$\left(0,b_{\infty}\right)$-subquadratic,

and we shall try to derive additional information.

%

\subsubsection{The General Case: Nontriviality.}

%

We assume that $H$ is

$\left(A_{\infty},B_{\infty}\right)$-sub\-qua\-dra\-tic at infinity,

for some constant symmetric matrices $A_{\infty}$ and $B_{\infty}$,

with $B_{\infty}-A_{\infty}$ positive definite. Set:

\begin{eqnarray}

\gamma :&=&{\rm smallest\ eigenvalue\ of}\ \ B_{\infty} - A_{\infty} \\

\lambda : &=& {\rm largest\ negative\ eigenvalue\ of}\ \

J \frac{d}{dt} +A_{\infty}\ .

\end{eqnarray}

Theorem 21 tells us that if $\lambda +\gamma < 0$, the boundary-value

problem:

\begin{equation}

\begin{array}{rcl}

\dot{x}&=&JH' (x)\\

x(0)&=&x (T)

\end{array}

\end{equation}

has at least one solution

$\overline{x}$, which is found by minimizing the dual

action functional:

\begin{equation}

\psi (u) = \int_{o}^{T} \left[\frac{1}{2}

\left(\Lambda_{o}^{-1} u,u\right) + N^{\ast} (-u)\right] dt

\end{equation}

on the range of $\Lambda$, which is a subspace $R (\Lambda)_{L}^{2}$

with finite codimension. Here

\begin{equation}

N(x) := H(x) - \frac{1}{2} \left(A_{\infty} x,x\right)

\end{equation}

is a convex function, and

\begin{equation}

N(x) \le \frac{1}{2}

\left(\left(B_{\infty} - A_{\infty}\right) x,x\right)

+ c\ \ \ \forall x\ .

\end{equation}

%

\begin{proposition}

Assume $H'(0)=0$ and $ H(0)=0$. Set:

\begin{equation}

\delta := \liminf_{x\to 0} 2 N (x) \left\|x\right\|^{-2}\ .

\label{2eq:one}

\end{equation}

If $\gamma < - \lambda < \delta$,

the solution $\overline{u}$ is non-zero:

\begin{equation}

\overline{x} (t) \ne 0\ \ \ \forall t\ .

\end{equation}

\end{proposition}

%

\begin{proof}

Condition (\ref{2eq:one}) means that, for every

$\delta ' > \delta$, there is some $\varepsilon > 0$ such that

\begin{equation}

\left\|x\right\| \le \varepsilon \Rightarrow N (x) \le

\frac{\delta '}{2} \left\|x\right\|^{2}\ .

\end{equation}

It is an exercise in convex analysis, into which we shall not go, to

show that this implies that there is an $\eta > 0$ such that

\begin{equation}

f\left\|x\right\| \le \eta

\Rightarrow N^{\ast} (y) \le \frac{1}{2\delta '}

\left\|y\right\|^{2}\ .

\label{2eq:two}

\end{equation}

\begin{figure}

\vspace{2.5cm}

\caption{This is the caption of the figure displaying a white eagle and

a white horse on a snow field}

\end{figure}

Since $u_{1}$ is a smooth function, we will have

$\left\|hu_{1}\right\|_\infty \le \eta$

for $h$ small enough, and inequality (\ref{2eq:two}) will hold,

yielding thereby:

\begin{equation}

\psi (hu_{1}) \le \frac{h^{2}}{2}

\frac{1}{\lambda} \left\|u_{1} \right\|_{2}^{2} + \frac{h^{2}}{2}

\frac{1}{\delta '} \left\|u_{1}\right\|^{2}\ .

\end{equation}

If we choose $\delta '$ close enough to $\delta$, the quantity

$\left(\frac{1}{\lambda} + \frac{1}{\delta '}\right)$

will be negative, and we end up with

\begin{equation}

\psi (hu_{1}) < 0\ \ \ \ \ {\rm for}\ \ h\ne 0\ \ {\rm small}\ .

\end{equation}

On the other hand, we check directly that $\psi (0) = 0$. This shows

that 0 cannot be a minimizer of $\psi$, not even a local one.

So $\overline{u} \ne 0$ and

$\overline{u} \ne \Lambda_{o}^{-1} (0) = 0$. \qed

\end{proof}

%

\begin{corollary}

Assume $H$ is $C^{2}$ and

$\left(a_{\infty},b_{\infty}\right)$-subquadratic at infinity. Let

$\xi_{1},\allowbreak\dots,\allowbreak\xi_{N}$ be the

equilibria, that is, the solutions of $H' (\xi ) = 0$.

Denote by $\omega_{k}$

the smallest eigenvalue of $H'' \left(\xi_{k}\right)$, and set:

\begin{equation}

\omega : = {\rm Min\,} \left\{\omega_{1},\dots,\omega_{k}\right\}\ .

\end{equation}

If:

\begin{equation}

\frac{T}{2\pi} b_{\infty} <

- E \left[- \frac{T}{2\pi}a_{\infty}\right] <

\frac{T}{2\pi}\omega

\label{2eq:three}

\end{equation}

then minimization of $\psi$ yields a non-constant $T$-periodic solution

$\overline{x}$.

\end{corollary}

%

We recall once more that by the integer part $E [\alpha ]$ of

$\alpha \in \bbbr$, we mean the $a\in \bbbz$

such that $a< \alpha \le a+1$. For instance,

if we take $a_{\infty} = 0$, Corollary 2 tells

us that $\overline{x}$ exists and is

non-constant provided that:

\begin{equation}

\frac{T}{2\pi} b_{\infty} < 1 < \frac{T}{2\pi}

\end{equation}

or

\begin{equation}

T\in \left(\frac{2\pi}{\omega},\frac{2\pi}{b_{\infty}}\right)\ .

\label{2eq:four}

\end{equation}

%

\begin{proof}

The spectrum of $\Lambda$ is $\frac{2\pi}{T} \bbbz +a_{\infty}$. The

largest negative eigenvalue $\lambda$ is given by

$\frac{2\pi}{T}k_{o} +a_{\infty}$,

where

\begin{equation}

\frac{2\pi}{T}k_{o} + a_{\infty} < 0

\le \frac{2\pi}{T} (k_{o} +1) + a_{\infty}\ .

\end{equation}

Hence:

\begin{equation}

k_{o} = E \left[- \frac{T}{2\pi} a_{\infty}\right] \ .

\end{equation}

The condition $\gamma < -\lambda < \delta$ now becomes:

\begin{equation}

b_{\infty} - a_{\infty} <

- \frac{2\pi}{T} k_{o} -a_{\infty} < \omega -a_{\infty}

\end{equation}

which is precisely condition (\ref{2eq:three}).\qed

\end{proof}

%

\begin{lemma}

Assume that $H$ is $C^{2}$ on $\bbbr^{2n} \setminus \{ 0\}$ and

that $H'' (x)$ is non-de\-gen\-er\-ate for any $x\ne 0$. Then any local

minimizer $\widetilde{x}$ of $\psi$ has minimal period $T$.

\end{lemma}

%

\begin{proof}

We know that $\widetilde{x}$, or

$\widetilde{x} + \xi$ for some constant $\xi

\in \bbbr^{2n}$, is a $T$-periodic solution of the Hamiltonian system:

\begin{equation}

\dot{x} = JH' (x)\ .

\end{equation}

There is no loss of generality in taking $\xi = 0$. So

$\psi (x) \ge \psi (\widetilde{x} )$

for all $\widetilde{x}$ in some neighbourhood of $x$ in

$W^{1,2} \left(\bbbr / T\bbbz ; \bbbr^{2n}\right)$.

But this index is precisely the index

$i_{T} (\widetilde{x} )$ of the $T$-periodic

solution $\widetilde{x}$ over the interval

$(0,T)$, as defined in Sect.~2.6. So

\begin{equation}

i_{T} (\widetilde{x} ) = 0\ .

\label{2eq:five}

\end{equation}

Now if $\widetilde{x}$ has a lower period, $T/k$ say,

we would have, by Corollary 31:

\begin{equation}

i_{T} (\widetilde{x} ) =

i_{kT/k}(\widetilde{x} ) \ge

ki_{T/k} (\widetilde{x} ) + k-1 \ge k-1 \ge 1\ .

\end{equation}

This would contradict (\ref{2eq:five}), and thus cannot happen.\qed

\end{proof}

%

\paragraph{Notes and Comments.}

The results in this section are a

refined version of \cite{2clar:eke};

the minimality result of Proposition

14 was the first of its kind.

To understand the nontriviality conditions, such as the one in formula

(\ref{2eq:four}), one may think of a one-parameter family

$x_{T}$, $T\in \left(2\pi\omega^{-1}, 2\pi b_{\infty}^{-1}\right)$

of periodic solutions, $x_{T} (0) = x_{T} (T)$,

with $x_{T}$ going away to infinity when $T\to 2\pi \omega^{-1}$,

which is the period of the linearized system at 0.

\begin{table}

\caption{This is the example table taken out of {\it The

\TeX{}book,} p.\,246}

\begin{center}

\begin{tabular}{r@{\quad}rl}

\hline

\multicolumn{1}{l}{\rule{0pt}{12pt}

Year}&\multicolumn{2}{l}{World population}\\[2pt]

\hline\rule{0pt}{12pt}

8000 B.C. & 5,000,000& \\

50 A.D. & 200,000,000& \\

1650 A.D. & 500,000,000& \\

1945 A.D. & 2,300,000,000& \\

1980 A.D. & 4,400,000,000& \\[2pt]

\hline

\end{tabular}

\end{center}

\end{table}

%

\begin{theorem} [Ghoussoub-Preiss]

Assume $H(t,x)$ is

$(0,\varepsilon )$-subquadratic at

infinity for all $\varepsilon > 0$, and $T$-periodic in $t$

\begin{equation}

H (t,\cdot )\ \ \ \ \ {\rm is\ convex}\ \ \forall t

\end{equation}

\begin{equation}

H (\cdot ,x)\ \ \ \ \ {\rm is}\ \ T{\rm -periodic}\ \ \forall x

\end{equation}

\begin{equation}

H (t,x)\ge n\left(\left\|x\right\|\right)\ \ \ \ \

{\rm with}\ \ n (s)s^{-1}\to \infty\ \ {\rm as}\ \ s\to \infty

\end{equation}

\begin{equation}

\forall \varepsilon > 0\ ,\ \ \ \exists c\ :\

H(t,x) \le \frac{\varepsilon}{2}\left\|x\right\|^{2} + c\ .

\end{equation}

Assume also that $H$ is $C^{2}$, and $H'' (t,x)$ is positive definite

everywhere. Then there is a sequence $x_{k}$, $k\in \bbbn$, of

$kT$-periodic solutions of the system

\begin{equation}

\dot{x} = JH' (t,x)

\end{equation}

such that, for every $k\in \bbbn$, there is some $p_{o}\in\bbbn$ with:

\begin{equation}

p\ge p_{o}\Rightarrow x_{pk} \ne x_{k}\ .

\end{equation}

\qed

\end{theorem}

%

\begin{example} [{{\rm External forcing}}]

Consider the system:

\begin{equation}

\dot{x} = JH' (x) + f(t)

\end{equation}

where the Hamiltonian $H$ is

$\left(0,b_{\infty}\right)$-subquadratic, and the

forcing term is a distribution on the circle:

\begin{equation}

f = \frac{d}{dt} F + f_{o}\ \ \ \ \

{\rm with}\ \ F\in L^{2} \left(\bbbr / T\bbbz; \bbbr^{2n}\right)\ ,

\end{equation}

where $f_{o} : = T^{-1}\int_{o}^{T} f (t) dt$. For instance,

\begin{equation}

f (t) = \sum_{k\in \bbbn} \delta_{k} \xi\ ,

\end{equation}

where $\delta_{k}$ is the Dirac mass at $t= k$ and

$\xi \in \bbbr^{2n}$ is a

constant, fits the prescription. This means that the system

$\dot{x} = JH' (x)$ is being excited by a

series of identical shocks at interval $T$.

\end{example}

%

\begin{definition}

Let $A_{\infty} (t)$ and $B_{\infty} (t)$ be symmetric

operators in $\bbbr^{2n}$, depending continuously on

$t\in [0,T]$, such that

$A_{\infty} (t) \le B_{\infty} (t)$ for all $t$.

A Borelian function

$H: [0,T]\times \bbbr^{2n} \to \bbbr$

is called

$\left(A_{\infty} ,B_{\infty}\right)$-{\it subquadratic at infinity}

if there exists a function $N(t,x)$ such that:

\begin{equation}

H (t,x) = \frac{1}{2} \left(A_{\infty} (t) x,x\right) + N(t,x)

\end{equation}

\begin{equation}

\forall t\ ,\ \ \ N(t,x)\ \ \ \ \

{\rm is\ convex\ with\ respect\ to}\ \ x

\end{equation}

\begin{equation}

N(t,x) \ge n\left(\left\|x\right\|\right)\ \ \ \ \

{\rm with}\ \ n(s)s^{-1}\to +\infty\ \ {\rm as}\ \ s\to +\infty

\end{equation}

\begin{equation}

\exists c\in \bbbr\ :\ \ \ H (t,x) \le

\frac{1}{2} \left(B_{\infty} (t) x,x\right) + c\ \ \ \forall x\ .

\end{equation}

If $A_{\infty} (t) = a_{\infty} I$ and

$B_{\infty} (t) = b_{\infty} I$, with

$a_{\infty} \le b_{\infty} \in \bbbr$,

we shall say that $H$ is

$\left(a_{\infty},b_{\infty}\right)$-subquadratic

at infinity. As an example, the function

$\left\|x\right\|^{\alpha}$, with

$1\le \alpha < 2$, is $(0,\varepsilon )$-subquadratic at infinity

for every $\varepsilon > 0$. Similarly, the Hamiltonian

\begin{equation}

H (t,x) = \frac{1}{2} k \left\|k\right\|^{2} +\left\|x\right\|^{\alpha}

\end{equation}

is $(k,k+\varepsilon )$-subquadratic for every $\varepsilon > 0$.

Note that, if $k<0$, it is not convex.

\end{definition}

%

\paragraph{Notes and Comments.}

The first results on subharmonics were

obtained by Rabinowitz in \cite{2rab}, who showed the existence of

infinitely many subharmonics both in the subquadratic and superquadratic

case, with suitable growth conditions on $H'$. Again the duality

approach enabled Clarke and Ekeland in \cite{2clar:eke:2} to treat the

same problem in the convex-subquadratic case, with growth conditions on

$H$ only.

Recently, Michalek and Tarantello (see Michalek, R., Tarantello, G.

\cite{2mich:tar} and Tarantello, G. \cite{2tar}) have obtained lower

bound on the number of subharmonics of period $kT$, based on symmetry

considerations and on pinching estimates, as in Sect.~5.2 of this

article.

%

% ---- Bibliography ----

%

\begin{thebibliography}{}

%

\bibitem[1980]{2clar:eke}

Clarke, F., Ekeland, I.:

Nonlinear oscillations and

boundary-value problems for Hamiltonian systems.

Arch. Rat. Mech. Anal. {\bf 78} (1982) 315--333

\bibitem[1981]{2clar:eke:2}

Clarke, F., Ekeland, I.:

Solutions p\'{e}riodiques, du

p\'{e}riode donn\'{e}e, des \'{e}quations hamiltoniennes.

Note CRAS Paris {\bf 287} (1978) 1013--1015

\bibitem[1982]{2mich:tar}

Michalek, R., Tarantello, G.:

Subharmonic solutions with prescribed minimal

period for nonautonomous Hamiltonian systems.

J. Diff. Eq. {\bf 72} (1988) 28--55

\bibitem[1983]{2tar}

Tarantello, G.:

Subharmonic solutions for Hamiltonian

systems via a $\bbbz_{p}$ pseudoindex theory.

Annali di Matematica Pura (to appear)

\bibitem[1985]{2rab}

Rabinowitz, P.:

On subharmonic solutions of a Hamiltonian system.

Comm. Pure Appl. Math. {\bf 33} (1980) 609--633

\end{thebibliography}

\clearpage

\addtocmark[2]{Author Index} % additional numbered TOC entry

\renewcommand{\indexname}{Author Index}

\printindex

\clearpage

\addtocmark[2]{Subject Index} % additional numbered TOC entry

\markboth{Subject Index}{Subject Index}

\renewcommand{\indexname}{Subject Index}

\input{subjidx.ind}

\end{document}