# Lagrangian actions on 3-body problems with two fixed centers

## Abstract

In this paper, we study the existence of figure "∞"-type periodic solution for 3-body problems with strong-force potentials and two fixed centers, and we also give some remarks in the case with Newtonian weak-force potentials.

Mathematical Subject Classification 2000: 34C15; 34C25; 70F10.

## 1 Introduction and Main Result

We assume two masses ${m}_{1}={m}_{2}=\frac{1}{2}$ are fixed at ${q}_{1}=\left(\frac{-1}{2},0\right)$ and ${q}_{2}=-{q}_{1}=\left(\frac{1}{2},0\right)$, the third mass m3 is affected by m1 and m2 and moving according to the Newton's second law and the general gravitational law [1, 2], then the position q(t) for m3 satisfies

${m}_{3}\stackrel{¨}{q}\left(t\right)=\frac{{m}_{1}{m}_{3}\alpha \left({q}_{1}-q\right)}{{\left|{q}_{1}-q\right|}^{\alpha +2}}+\frac{{m}_{2}{m}_{3}\alpha \left({q}_{2}-q\right)}{{\left|{q}_{2}-q\right|}^{\alpha +2}}$
(1.1)

Equivalently,

$\stackrel{¨}{q}\left(t\right)=\frac{\alpha }{2}\left[\frac{{q}_{1}-q}{{\left|{q}_{1}-q\right|}^{\alpha +2}}+\frac{{q}_{2}-q}{{\left|{q}_{2}-q\right|}^{\alpha +2}}\right]$
(1.2)
$\stackrel{¨}{q}\left(t\right)=\frac{\partial U\left(q\right)}{\partial q}$
(1.3)
$\mathsf{\text{Where}}\phantom{\rule{2.77695pt}{0ex}}\alpha >0,U\left(q\right)=\frac{1/2}{{\left|q-{q}_{1}\right|}^{\alpha }}+\frac{1/2}{{\left|q-{q}_{2}\right|}^{\alpha }}.$
(1.4)

For the case α = 1, Euler  studied (1.1)-(1.3), but didn't use variational methods to study periodic solutions.

Here we want to use variational minimizing method to look for periodic solution for m3 which winds around q1 and q2, let

$f\left(q\right)=\underset{0}{\overset{1}{\int }}\left[\frac{1}{2}{\left|\stackrel{·}{q}\right|}^{2}+\frac{1/2}{{\left|q-{q}_{1}\right|}^{\alpha }}+\frac{1/2}{{\left|q-{q}_{2}\right|}^{\alpha }}\right]\phantom{\rule{2.77695pt}{0ex}}dt,$
(1.5)
$q\in \Lambda =\left\{\begin{array}{c}q\in {W}^{1,2}\left(ℝ/ℤ,\phantom{\rule{2.77695pt}{0ex}}\phantom{\rule{2.77695pt}{0ex}}{ℝ}^{2}\right),\phantom{\rule{1em}{0ex}}q\left(t\right)\ne {q}_{1},\phantom{\rule{2.77695pt}{0ex}}\phantom{\rule{2.77695pt}{0ex}}{q}_{2},\hfill \\ q\left(t+\frac{1}{2}\right)=\left(\begin{array}{cc}-1\hfill & \hfill 0\hfill \\ 0\hfill & \hfill 1\hfill \end{array}\right)\phantom{\rule{2.77695pt}{0ex}}q\left(t\right),\phantom{\rule{1em}{0ex}}q\left(-t\right)=-q\left(t\right),\hfill \\ \text{deg}\left(q-{q}_{1}\right)=1,\phantom{\rule{2.77695pt}{0ex}}\text{deg}\left(q-{q}_{2}\right)=-1\hfill \end{array}\right\}$
(1.6)

Theorem 1.1 For α ≥ 2, the minimizer of f(q) on $\overline{\Lambda }$ does exist and is non-collision "∞"-type periodic solution of (1.1)-(1.3).(See Figure 1)

## 2 The Proof of Theorem 1.1

Using Palais'S symmetrical Principle , it's easy to prove the following variational Lemma:

Lemma 2.1 The critical point of f(q) in Λ is the noncollision periodic solution winding around q1 counter-clockwise and q2 clockwise one time during one period.

Lemma 2.2 If x W1,2 (/, 2) and t0 [0,1], s.t. x(t0) = 0, if α ≥ 2 and a > 0, then

$\underset{0}{\overset{1}{\int }}\left[\frac{1}{2}{\left|ẋ\right|}^{2}+\frac{a}{{\left|x\right|}^{\alpha }}\right]\phantom{\rule{2.77695pt}{0ex}}dt=+\infty$
(2.1)

It's easy to see

Lemma 2.3$\overline{\Lambda }$ is a weakly closed subset of the Hilbert space W1,2(/, 2).

Lemma 2.4 f(q) is coercive and weakly lower-semicontinuous on the closure $\overline{\Lambda }$ of Λ.

Proof. By q(-t) = -q(t) and q(t) W1,2(/, 2), we have $\underset{0}{\overset{1}{\int }}q\left(t\right)dt=0$. By Wirtinger's inequality, we know f(q) is coercive. By Sobolev's embedding Theorem and Fatou's Lemma, f is weakly lower-semi-continuous on the weakly closed set $\overline{\Lambda }$ of W1,2.

Lemma 2.5 Let X be a reflexive Banach space,M X be weakly closed subset,f : MR be weakly lower semi-continous and coercive (f(x) → +∞ as x → +∞), then f attains its infimum on M.

According to Lemmas 2.1-2.5, we know that f(q) attains its infimum on $\overline{\Lambda }$ and the minimizer of f(q) on $\overline{\Lambda }$ is collision-free since if let x1 = q - q1, x2 = q - q2, then

$\begin{array}{ll}f\left(q\right)\hfill & =\underset{0}{\overset{1}{\int }}\left[\frac{1}{2}|\stackrel{˙}{q}-{\stackrel{˙}{q}}_{1}{|}^{2}+\frac{1}{|q-{q}_{1}{|}^{\alpha }}\right]dt=\underset{0}{\overset{1}{\int }}\left[\frac{1}{2}|\stackrel{˙}{q}-{\stackrel{˙}{q}}_{2}{|}^{2}+\frac{1}{|q-{q}_{2}{|}^{\alpha }}\right]dt\hfill \\ =\underset{0}{\overset{1}{\int }}\left[\frac{1}{2}|{\stackrel{˙}{x}}_{1}{|}^{2}+\frac{1}{|{x}_{1}{|}^{\alpha }}\right]dt=\underset{0}{\overset{1}{\int }}\left[\frac{1}{2}|{\stackrel{˙}{x}}_{2}{|}^{2}+\frac{1}{|{x}_{2}{|}^{\alpha }}\right]dt\hfill \end{array}$
(2.2)

So if the minimizer of f(q) on $\overline{\Lambda }$ has collision at some moment, then Gordon's Lemma tell us the minimum value is +∞ which is a contradiction.

The most interesting case α = 1 is the case for Newtonian potential, we try to prove the minimizer is collision-free, but it seems very difficult, here we give some remarks.

Lemma 2.6 If y(0) = 0 and 2k is an even positive integer, then

$\underset{0}{\overset{1}{\int }}{y}^{2k}dx\le c\underset{0}{\overset{1}{\int }}{ẏ}^{2k}dx,$
(2.3)

where

$c=\frac{1}{2k-1}{\left(\frac{2k}{\pi }\text{sin}\left(\frac{\pi }{2k}\right)\right)}^{2k}.$
(2.4)

There is equality only for a certain hyperelliptic curve.

Now we estimate the lower bound of the Lagrangian action f(q) on "∞"-type collisionorbits. Since ${q}_{2}=-{q}_{1}=\left(\frac{1}{2},0\right)$ and $q\left(t+\frac{1}{2}\right)=\left(\begin{array}{cc}-1\hfill & \hfill 0\hfill \\ 0\hfill & \hfill 1\hfill \end{array}\right)\phantom{\rule{2.77695pt}{0ex}}q\left(t\right)$, so

$\underset{0}{\overset{1}{\int }}\frac{dt}{\left|q-{q}_{1}\right|}=\underset{0}{\overset{1}{\int }}\frac{dt}{\left|q-{q}_{2}\right|},$
(2.5)
$\begin{array}{ll}\hfill f\left(q\right)& =\underset{0}{\overset{1}{\int }}\left[\frac{1}{2}{\left|\stackrel{·}{q}\right|}^{2}+\frac{1/2}{\left|q-{q}_{1}\right|}+\frac{1/2}{\left|q-{q}_{2}\right|}\right]\phantom{\rule{2.77695pt}{0ex}}dt\phantom{\rule{2em}{0ex}}\\ =\underset{0}{\overset{1}{\int }}\left[\frac{1}{2}{\left|\stackrel{·}{q}\right|}^{2}+\frac{1}{\left|q-{q}_{1}\right|}\right]\phantom{\rule{2.77695pt}{0ex}}dt\phantom{\rule{2em}{0ex}}\\ =\underset{0}{\overset{1}{\int }}\left[\frac{1}{2}{\left|\stackrel{·}{q}-{\stackrel{·}{q}}_{1}\right|}^{2}+\frac{1}{\left|q-{q}_{1}\right|}\right]\phantom{\rule{2.77695pt}{0ex}}dt\phantom{\rule{2em}{0ex}}\end{array}$
(2.6)

If q(t) collides with q1 at some moment t0 [0,1], without loss of generality, we assume t0 = 0, then q(0) - q1 = 0, we let x(t) = q(t) - q1, y(t) = |x(t)|, then x(0) = 0,y(0) = 0. By Jensen's inequality and Hardy-Littlewood-pólya inequality , we have

$\begin{array}{ll}\hfill f\left(q\right)& =\frac{1}{2}\underset{0}{\overset{1}{\int }}\left({\left|ẋ\right|}^{2}dt+\underset{0}{\overset{1}{\int }}\frac{dt}{\left|x\right|}\right)\phantom{\rule{2em}{0ex}}\\ \ge \frac{1}{2}\underset{0}{\overset{1}{\int }}{\left[\frac{d}{dt}\left|x\right|\right]}^{2}dt+{1}^{3/2}{\left(\underset{0}{\overset{1}{\int }}{\left|x\right|}^{2}dt\right)}^{-1/2}\phantom{\rule{2em}{0ex}}\\ =\frac{1}{2}\underset{0}{\overset{1}{\int }}{ẏ}^{2}dt+{\left(\underset{0}{\overset{1}{\int }}{y}^{2}dt\right)}^{-1/2}\phantom{\rule{2em}{0ex}}\\ \ge \frac{1}{2}{\left(\frac{\pi }{2}\right)}^{2}\underset{0}{\overset{1}{\int }}{y}^{2}dt+{\left(\underset{0}{\overset{1}{\int }}{y}^{2}dt\right)}^{-1/2}\phantom{\rule{2em}{0ex}}\end{array}$
(2.7)

Let $\sqrt{{\int }_{0}^{1}{y}^{2}dt}=s\ge 0,\phi \left(s\right)=\frac{{\pi }^{2}}{8}{s}^{2}+{s}^{-1}$, then ${\phi }^{″}\left(s\right)=\frac{{\pi }^{2}}{4}+2{s}^{-3}>0$, that is φ is strictly convex.

Let φ'(s) = 0, we solve it to get ${s}_{0}={\left(\frac{{\pi }^{2}}{4}\right)}^{-1/3}$ is the critical point for φ(s), and $\phi \left({s}_{0}\right)=\frac{3}{2}{\left(\frac{\pi }{2}\right)}^{\frac{2}{3}}$, which is the maximum value for φ(s) on s > 0 since φ is convex and φ(s) → +∞ as s → 0+.

If we can find the test orbit $\stackrel{̃}{q}\left(t\right)\in \Lambda$ such that

$f\left(\stackrel{̃}{q}\left(t\right)\right)<\frac{3}{2}{\left(\frac{\pi }{2}\right)}^{2/3}$
(2.8)

then the minimizer of f(q) on $\overline{\Lambda }$ is collision-free.

## References

1. Gordon W: A minimizing property of Keplerian orbits. Am J Math 1977, 99: 961-971. 10.2307/2373993

2. Siegel C, Moser J: Lectures on Celestial Mechanics. Springer, Berlin; 1971.

3. Euler M: De motu coproris ad duo centra virium fixa attracti. Nov Commun Acad Sci Imp Petrop 1766, 10: 207-242.

4. Euler M: De motu coproris ad duo centra virium fixa attracti. Nov Commun Acad Sci Imp Petrop 1767, 11: 152-184.

5. Euler M: Probleme un corps etant attire en raison reciproque quarree des distances vers deux points fixes donnes trouver les cas ou la courbe decrite par ce corps sera algebrique. Hist Acad R Sci Bell Lett Berlin 1767, 2: 228-249.

6. Palais R: The principle of symmetric criticality. Commun Math Phys 1979, 69: 19-30. 10.1007/BF01941322

7. Gordon W: Conservative dynamical systems involving strong forces. Trans AMS 1975, 204: 113-135.

8. Ambrosetti A, Coti Zelati V: Periodic Solutions for Singular Lagrangian Systems. Springer, Boston; 1993.

9. Hardy G, Littlewood JE, Pólya G: Inequalities. 2nd edition. Cambridge University Press, Cambridge; 1952.

## Acknowledgements

The authors would like to thank the anonymous referees for their valuable suggestions which improve this work. This work was supported by Scientific Research Fund of Sichuan Provincial Education Department (11ZA172).

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Correspondence to Sheng He.

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Wang, Xr., He, S. Lagrangian actions on 3-body problems with two fixed centers. Bound Value Probl 2012, 28 (2012). https://doi.org/10.1186/1687-2770-2012-28 