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2015 | OriginalPaper | Buchkapitel

5. Calculus of Variations, Conjugate Points and Morse Index

verfasst von : Franco Cardin

Erschienen in: Elementary Symplectic Topology and Mechanics

Verlag: Springer International Publishing

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Abstract

Let us re-examine the classical conditions of Calculus of Variations geared to obtain a strong minimum (in the topology of the uniform convergence) for an arbitrary Lagrangian function \(L(t,q,\dot{q})\), convex in the velocities \(\dot{q}\). The result can be obtained in the geometric setup of symplectic geometry using the Hamiltonian description of the problem: the joint use of the theory of Poincaré-Cartan Integral Invariant and Young Inequality rapidly leads to the thesis. The Morse index of stationary curves is discussed in the mechanical case.

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Fußnoten
1
The above uniform convexity may be weakened by the so-called Tonelli condition: (i) L is (simply) \(\dot{q}\)-convex and (ii) L is \(\dot{q}\)-superlinear at infinity: \(\lim _{\vert \dot{q}\vert \rightarrow +\infty }\frac{L(q,\dot{q})} {\vert \dot{q}\vert } = \infty \). In such a case also \(\mathcal{H}\) is Tonelli, in the p variables.
 
2
E.g. homotopically related each other by a continuous deformation over Λ n+1 .
 
3
In a unique way, thanks to the transversality condition.
 
4
See below: \(\varGamma _{t_{0},t_{1}}^{q_{0},q_{1}}\).
 
5
To be precise, we should write:
$$\displaystyle\begin{array}{rcl} & & [t_{0},t_{1}] \times B^{n} \ni (t,v)\longmapsto (t,q(t,v); -\mathcal{H}(t,q(t,v),p(t,v)),p(t,v)) {}\\ & & \quad =\overbrace{ (t,q(t); -\mathcal{H}(t,q(t),p(t)),p(t))}^{\gamma (t)} {}\\ & & \qquad + (0,Q(t,v); \mathcal{H}(t,q(t),p(t)) -\mathcal{H}(t,q(t) + Q(t,v),p(t) + P(t,v)),P(t,v)) \in \mathbb{R}^{2n+2}, {}\\ & & \text{where}\ \ \ x(t,v) = x(t) + f(t,v) = (q(t) + Q(t,v),p(t) + P(t,v)), {}\\ & & \qquad f(t, 0) = (Q(t, 0),P(t, 0)) = 0. {}\\ \end{array}$$
 
6
We can see that the essential part of the system (5.15) for γ(t) is given by \(\dot{x}(t) = \mathbb{E}\nabla _{x}\mathcal{H}\big(t,x(t)\big)\), since the evolution of q 0(t) = t and \(p_{0}(t) = -\mathcal{H}\) is a trivial consequence.
 
7
Parameters t and λ are independent between them.
 
8
The same regularity (5.40) of L, thanks to Proposition 5.3.
 
9
The n × n-matrices \(A,B,\dot{A},\dot{B}\) are evalued for initial and final times τ α and τ α+1.
 
10
Continuous, but not differentiable.
 
11
To be honest, the introduction proposed above of functions q(⋅ ) twice-differentiable is only needed to establish the equivalence between Lagrange equations and Gateaux-stationary points for Hamilton’s functional. We overcome this a priori strange requirement by introducing the DuBois-Reymond Lemma.
 
12
\(h \in H_{0}^{1}([a,b], \mathbb{R}^{n})\;\Longleftrightarrow\;h \in H^{1}([a,b], \mathbb{R}^{n})\ \text{and}\ h(a) = 0 = h(b)\).
 
13
Note that J′[q]h and J″[q](h,h) are the first and second Fréchet derivatives with respect the norm \(\|\cdot \|.\)
 
14
See proposition 2.3 in [1].
 
15
See e.g. [89].
 
16
\(h \in H_{0}^{1}([t_{0},t_{1}], \mathbb{R}^{n})\;\Longleftrightarrow\;h \in H^{1}([t_{0},t_{1}], \mathbb{R}^{n})\ \text{e}\ h(t_{0}) = 0 = h(t_{1})\).
 
17
Note that \(\alpha _{2} =\alpha _{1}\frac{(t_{1}-t_{0})^{2}} {2}\).
 
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Metadaten
Titel
Calculus of Variations, Conjugate Points and Morse Index
verfasst von
Franco Cardin
Copyright-Jahr
2015
DOI
https://doi.org/10.1007/978-3-319-11026-4_5