Three-body problem explained

In physics, specifically classical mechanics, the three-body problem involves taking the initial positions and velocities (or momenta) of three point masses that orbit each other in space and calculating their subsequent trajectories using Newton's laws of motion and Newton's law of universal gravitation.^[1]

Unlike the two-body problem, the three-body problem has no general closed-form solution.^[1] When three bodies orbit each other, the resulting dynamical system is chaotic for most initial conditions, and the only way to predict the motions of the bodies is to calculate them using numerical methods.

The three-body problem is a special case of the -body problem. Historically, the first specific three-body problem to receive extended study was the one involving the Earth, the Moon, and the Sun.^[2] In an extended modern sense, a three-body problem is any problem in classical mechanics or quantum mechanics that models the motion of three particles.

Mathematical description

The mathematical statement of the three-body problem can be given in terms of the Newtonian equations of motion for vector positions

r _{i} = (x _{i,} y _{i,} z _{i)} of three gravitationally interacting bodies with masses m _{i} :

^3

- G m_3 \frac

^3

, \\ \ddot\mathbf_ &= -G m_3 \frac

^3

- G m_1 \frac

^3

, \\ \ddot\mathbf_ &= -G m_1 \frac

^3

- G m_2 \frac

^3

.\endwhere G is the gravitational constant.^{[3] [4]} This is a set of nine second-order differential equations. The problem can also be stated equivalently in the Hamiltonian formalism, in which case it is described by a set of 18 first-order differential equations, one for each component of the positions r _{i} and momenta p _{i} :

$$\backslash frac\; =\; \backslash frac,\; \backslash qquad\; \backslash frac\; =\; -\backslash frac,$$

where

l{H} is the Hamiltonian:

$$\backslash mathcal\; =\; -\backslash frac$$

-\frac -\frac + \frac + \frac + \frac.

In this case

l{H} is simply the total energy of the system, gravitational plus kinetic.

Restricted three-body problem

In the restricted three-body problem, a body of negligible mass (the "planetoid") moves under the influence of two massive bodies. Having negligible mass, the planetoid exerts force on the two massive bodies that may be neglected; therefore the resulting system can be analyzed and described as a two-body motion problem.^{[3] [5]} With respect to a rotating reference frame, the two co-orbiting bodies are stationary, and the third can be stationary as well at the Lagrangian points, or move around them, for instance on a horseshoe orbit. It can be useful to consider the effective potential. Usually this two-body motion is taken to consist of circular orbits around the center of mass, and the planetoid is assumed to move in the plane defined by the circular orbits.

The restricted three-body problem is easier to analyze theoretically than the full problem. It is of practical interest as well since it accurately describes many real-world problems, the most important example being the Earth–Moon–Sun system. For these reasons, it has occupied an important role in the historical development of the three-body problem.^[6]

Mathematically, the problem is stated as follows. Let

m _{1,2} be the masses of the two massive bodies, with (planar) coordinates (x _{1,} y _{1)} and (x _{2,} y _{2)} , and let (x, y) be the coordinates of the planetoid. For simplicity, choose units such that the distance between the two massive bodies, as well as the gravitational constant, are both equal to 1 . Then, the motion of the planetoid is given by

$$\backslash begin\backslash frac\; =\; -m\_1\; \backslash frac\; -\; m\_2\; \backslash frac,\; \backslash \backslash \backslash frac\; =\; -m\_1\; \backslash frac\; -\; m\_2\; \backslash frac,\backslash end$$

where

r _{i} = \sqrt{(x - x ^{2} _{i)} + (y - y ^{2 }} _{i)} . In this form the equations of motion carry an explicit time dependence through the coordinates x _{i(t),} y _{i(t)} . However, this time dependence can be removed through a transformation to a rotating reference frame, which simplifies any subsequent analysis.^[7]

Solutions

General solution

There is no general closed-form solution to the three-body problem.^[1] In other words, it does not have a general solution that can be expressed in terms of a finite number of standard mathematical operations. Moreover, the motion of three bodies is generally non-repeating, except in special cases.^[8]

However, in 1912 the Finnish mathematician Karl Fritiof Sundman proved that there exists an analytic solution to the three-body problem in the form of a Puiseux series, specifically a power series in terms of powers of .^[9] This series converges for all real, except for initial conditions corresponding to zero angular momentum. In practice, the latter restriction is insignificant since initial conditions with zero angular momentum are rare, having Lebesgue measure zero.

An important issue in proving this result is the fact that the radius of convergence for this series is determined by the distance to the nearest singularity. Therefore, it is necessary to study the possible singularities of the three-body problems. As is briefly discussed below, the only singularities in the three-body problem are binary collisions (collisions between two particles at an instant) and triple collisions (collisions between three particles at an instant).

Collisions of any number are somewhat improbable, since it has been shown that they correspond to a set of initial conditions of measure zero. But there is no criterion known to be put on the initial state in order to avoid collisions for the corresponding solution. So Sundman's strategy consisted of the following steps:

1. Using an appropriate change of variables to continue analyzing the solution beyond the binary collision, in a process known as regularization.
2. Proving that triple collisions only occur when the angular momentum vanishes. By restricting the initial data to, he removed all real singularities from the transformed equations for the three-body problem.
3. Showing that if, then not only can there be no triple collision, but the system is strictly bounded away from a triple collision. This implies, by Cauchy's existence theorem for differential equations, that there are no complex singularities in a strip (depending on the value of) in the complex plane centered around the real axis (related to the Cauchy–Kovalevskaya theorem).
4. Find a conformal transformation that maps this strip into the unit disc. For example, if (the new variable after the regularization) and if, then this map is given by $$\backslash sigma\; =\; \backslash frac.$$

This finishes the proof of Sundman's theorem.

The corresponding series converges extremely slowly. That is, obtaining a value of meaningful precision requires so many terms that this solution is of little practical use. Indeed, in 1930, David Beloriszky calculated that if Sundman's series were to be used for astronomical observations, then the computations would involve at least 10 terms.^[10]

Special-case solutions

In 1767, Leonhard Euler found three families of periodic solutions in which the three masses are collinear at each instant.

In 1772, Lagrange found a family of solutions in which the three masses form an equilateral triangle at each instant. Together with Euler's collinear solutions, these solutions form the central configurations for the three-body problem. These solutions are valid for any mass ratios, and the masses move on Keplerian ellipses. These four families are the only known solutions for which there are explicit analytic formulae. In the special case of the circular restricted three-body problem, these solutions, viewed in a frame rotating with the primaries, become points called Lagrangian points and labeled L₁, L₂, L₃, L₄, and L₅, with L₄ and L₅ being symmetric instances of Lagrange's solution.

In work summarized in 1892–1899, Henri Poincaré established the existence of an infinite number of periodic solutions to the restricted three-body problem, together with techniques for continuing these solutions into the general three-body problem.

In 1893, Meissel stated what is now called the Pythagorean three-body problem: three masses in the ratio 3:4:5 are placed at rest at the vertices of a 3:4:5 right triangle, with the heaviest body at the right angle and the lightest at the smaller acute angle. Burrau^[11] further investigated this problem in 1913. In 1967 Victor Szebehely and C. Frederick Peters established eventual escape of the lightest body for this problem using numerical integration, while at the same time finding a nearby periodic solution.^[12]

In the 1970s, Michel Hénon and Roger A. Broucke each found a set of solutions that form part of the same family of solutions: the Broucke–Hénon–Hadjidemetriou family. In this family, the three objects all have the same mass and can exhibit both retrograde and direct forms. In some of Broucke's solutions, two of the bodies follow the same path.^[13]

In 1993, physicist Cris Moore at the Santa Fe Institute found a zero angular momentum solution with three equal masses moving around a figure-eight shape.^[14] In 2000, mathematicians Alain Chenciner and Richard Montgomery proved its formal existence.^{[15] [16]} The solution has been shown numerically to be stable for small perturbations of the mass and orbital parameters, which makes it possible for such orbits to be observed in the physical universe. But it has been argued that this is unlikely since the domain of stability is small. For instance, the probability of a binary–binary scattering event resulting in a figure-8 orbit has been estimated to be a small fraction of a percent.^[17]

In 2013, physicists Milovan Šuvakov and Veljko Dmitrašinović at the Institute of Physics in Belgrade discovered 13 new families of solutions for the equal-mass zero-angular-momentum three-body problem.^{[8] [13]}

In 2015, physicist Ana Hudomal discovered 14 new families of solutions for the equal-mass zero-angular-momentum three-body problem.^[18]

In 2017, researchers Xiaoming Li and Shijun Liao found 669 new periodic orbits of the equal-mass zero-angular-momentum three-body problem.^[19] This was followed in 2018 by an additional 1,223 new solutions for a zero-angular-momentum system of unequal masses.^[20]

In 2018, Li and Liao reported 234 solutions to the unequal-mass "free-fall" three-body problem.^[21] The free-fall formulation starts with all three bodies at rest. Because of this, the masses in a free-fall configuration do not orbit in a closed "loop", but travel forward and backward along an open "track".

In 2023, Ivan Hristov, Radoslava Hristova, Dmitrašinović and Kiyotaka Tanikawa published a search for "periodic free-fall orbits" three-body problem, limited to the equal-mass case, and found 12,409 distinct solutions.^[22]

Numerical approaches

Using a computer, the problem may be solved to arbitrarily high precision using numerical integration although high precision requires a large amount of CPU time. There have been attempts of creating computer programs that numerically solve the three-body problem (and by extension, the n-body problem) involving both electromagnetic and gravitational interactions, and incorporating modern theories of physics such as special relativity.^[23] In addition, using the theory of random walks, an approximate probability of different outcomes may be computed.^{[24] [25]}

History

The gravitational problem of three bodies in its traditional sense dates in substance from 1687, when Isaac Newton published his Philosophiæ Naturalis Principia Mathematica, when Newton was trying to figure out if any long term stability is possible, especially the system of our Earth, the Moon, and the Sun. He was guided under the major Renaissance astronomers Nicolaus Copernicus, Tycho Brahe and Johannes Kepler to the beginning of the gravitational three-body problem.^[26] In Proposition 66 of Book 1 of the Principia, and its 22 Corollaries, Newton took the first steps in the definition and study of the problem of the movements of three massive bodies subject to their mutually perturbing gravitational attractions. In Propositions 25 to 35 of Book 3, Newton also took the first steps in applying his results of Proposition 66 to the lunar theory, the motion of the Moon under the gravitational influence of Earth and the Sun.^[27] Later, this problem was also applied to other planets' interactions with the Earth and the Sun.

The physical problem was first addressed by Amerigo Vespucci and subsequently by Galileo Galilei, as well as Simon Stevin, but they did not realize what they contributed. Though Galileo determined that the speed of fall of all bodies changes uniformly and in the same way, he did not apply it to planetary motions. Whereas in 1499, Vespucci used knowledge of the position of the Moon to determine his position in Brazil.^[28] It became of technical importance in the 1720s, as an accurate solution would be applicable to navigation, specifically for the determination of longitude at sea, solved in practice by John Harrison's invention of the marine chronometer. However the accuracy of the lunar theory was low, due to the perturbing effect of the Sun and planets on the motion of the Moon around Earth.

Jean le Rond d'Alembert and Alexis Clairaut, who developed a longstanding rivalry, both attempted to analyze the problem in some degree of generality; they submitted their competing first analyses to the Académie Royale des Sciences in 1747.^[29] It was in connection with their research, in Paris during the 1740s, that the name "three-body problem" (French: Problème des trois Corps) began to be commonly used. An account published in 1761 by Jean le Rond d'Alembert indicates that the name was first used in 1747.^[30]

From the end of the 19th century to early 20th century, the approach to solve the three-body problem with the usage of short-range attractive two-body forces was developed by scientists, which offered P.F. Bedaque, H.-W. Hammer and U. van Kolck an idea to renormalize the short-range three-body problem, providing scientists a rare example of a renormalization group limit cycle at the beginning of the 21st century.^[31] George William Hill worked on the restricted problem in the late 19th century with an application of motion of Venus and Mercury.^[32]

At the beginning of the 20th century, Karl Sundman approached the problem mathematically and systematically by providing a functional theoretical proof to the problem valid for all values of time. It was the first time scientists theoretically solved the three-body problem. However, because there was not a qualitative enough solution of this system, and it was too slow for scientists to practically apply it, this solution still left some issues unresolved.^[33] In the 1970s, implication to three-body from two-body forces had been discovered by V. Efimov, which was named the Efimov effect.^[34]

In 2017, Shijun Liao and Xiaoming Li applied a new strategy of numerical simulation for chaotic systems called the clean numerical simulation (CNS), with the use of a national supercomputer, to successfully gain 695 families of periodic solutions of the three-body system with equal mass.^[35]

In 2019, Breen et al. announced a fast neural network solver for the three-body problem, trained using a numerical integrator.^[36]

In September 2023, several possible solutions have been found to the problem according to reports.^{[37] [38]}

Other problems involving three bodies

The term "three-body problem" is sometimes used in the more general sense to refer to any physical problem involving the interaction of three bodies.

A quantum-mechanical analogue of the gravitational three-body problem in classical mechanics is the helium atom,in which a helium nucleus and two electrons interact according to the inverse-square Coulomb interaction. Like thegravitational three-body problem, the helium atom cannot be solved exactly.^[39]

In both classical and quantum mechanics, however, there exist nontrivial interaction laws besides the inverse-square force that do lead to exact analytic three-body solutions. One such model consists of a combination of harmonic attraction and a repulsive inverse-cube force.^[40] This model is considered nontrivial since it is associated with a set of nonlinear differential equations containing singularities (compared with, e.g., harmonic interactions alone, which lead to an easily solved system of linear differential equations). In these two respects it is analogous to (insoluble) models having Coulomb interactions, and as a result has been suggested as a tool for intuitively understanding physical systems like the helium atom.^{[40] [41]}

Within the point vortex model, the motion of vortices in a two-dimensional ideal fluid is described by equations of motion that contain only first-order time derivatives. I.e. in contrast to Newtonian mechanics, it is the velocity and not the acceleration that is determined by their relative positions. As a consequence, the three-vortex problem is still integrable,^[42] while at least four vortices are required to obtain chaotic behavior.^[43] One can draw parallels between the motion of a passive tracer particle in the velocity field of three vortices and the restricted three-body problem of Newtonian mechanics.^[44]

The gravitational three-body problem has also been studied using general relativity. Physically, a relativistic treatment becomes necessary in systems with very strong gravitational fields, such as near the event horizon of a black hole. However, the relativistic problem is considerably more difficult than in Newtonian mechanics, and sophisticated numerical techniques are required.Even the full two-body problem (i.e. for arbitrary ratio of masses) does not have a rigorous analytic solution in general relativity.^[45]

-body problem

The three-body problem is a special case of the -body problem, which describes how objects move under one of the physical forces, such as gravity. These problems have a global analytical solution in the form of a convergent power series, as was proven by Karl F. Sundman for and by Qiudong Wang for (see -body problem for details). However, the Sundman and Wang series converge so slowly that they are useless for practical purposes;^[46] therefore, it is currently necessary to approximate solutions by numerical analysis in the form of numerical integration or, for some cases, classical trigonometric series approximations (see -body simulation). Atomic systems, e.g. atoms, ions, and molecules, can be treated in terms of the quantum -body problem. Among classical physical systems, the -body problem usually refers to a galaxy or to a cluster of galaxies; planetary systems, such as stars, planets, and their satellites, can also be treated as -body systems. Some applications are conveniently treated by perturbation theory, in which the system is considered as a two-body problem plus additional forces causing deviations from a hypothetical unperturbed two-body trajectory.

See also

• Few-body systems
• Galaxy formation and evolution
• Gravity assist
• Lagrange point
• Low-energy transfer
• Michael Minovitch
• -body simulation
• Symplectic integrator
• Sitnikov problem
• Two-body problem
  • Synodic reference frame Triple star system
• The Three-Body Problem (novel)
• 3 Body Problem (TV series)

Further reading

• Book: Aarseth, S. J. . Gravitational n-Body Simulations . 2003 . Cambridge University Press . New York . 978-0-521-43272-6 .
• Bagla . J. S. . Cosmological N-body simulation: Techniques, scope and status . 2005 . . 88 . 1088–1100 . astro-ph/0411043 . 2005CSci...88.1088B .
• Chambers . J. E. . Wetherill . G. W. . Making the Terrestrial Planets: N-Body Integrations of Planetary Embryos in Three Dimensions . 1998 . . 136 . 2 . 304–327 . 10.1006/icar.1998.6007 . 1998Icar..136..304C . 10.1.1.64.7797 .
• Efstathiou . G. . Davis . M. . White . S. D. M. . Frenk . C. S. . Numerical techniques for large cosmological N-body simulations . 1985 . . 57 . 241–260 . 10.1086/191003 . 1985ApJS...57..241E . free .
• Hulkower . Neal D. . The Zero Energy Three Body Problem . . 27 . 3 . 1978 . 409–447 . 10.1512/iumj.1978.27.27030. 1978IUMJ...27..409H . free .
• Hulkower . Neal D. . Central Configurations and Hyperbolic-Elliptic Motion in the Three-Body Problem . . 21 . 1980 . 1 . 37–41 . 10.1007/BF01230244 . 1980CeMec..21...37H . 123404551 .
• Li . Xiaoming . Liao . Shijun . 1312.6796 . On the stability of the three classes of Newtonian three-body planar periodic orbits . 2014 . . 57 . 11 . 2121–2126 . 10.1007/s11433-014-5563-5 . 2014SCPMA..57.2121L . 73682020 .
• Moore . Cristopher . 1993 . Braids in Classical Dynamics . Physical Review Letters . 70 . 24 . 3675–3679 . 1993PhRvL..70.3675M . 10.1103/PhysRevLett.70.3675 . 10053934 . 2016-01-01 . 2018-10-08 . https://web.archive.org/web/20181008213647/http://tuvalu.santafe.edu/~moore/braids-prl.pdf .
• Book: Poincaré, H. . 1967 . New Methods of Celestial Mechanics . registration . 3 vol. English translated . American Institute of Physics . 978-1-56396-117-5 .
• Šuvakov . Milovan . Dmitrašinović . V. . 1303.0181 . Three Classes of Newtonian Three-Body Planar Periodic Orbits . 2013 . . 110 . 10 . 114301 . 10.1103/PhysRevLett.110.114301 . 25166541 . 2013PhRvL.110k4301S . 118554305 .

External links

• Alain . Chenciner . 2007 . Three body problem . . 2 . 10 . 2111 . 10.4249/scholarpedia.2111 . 2007SchpJ...2.2111C . free .
• Physicists Discover a Whopping 13 New Solutions to Three-Body Problem (Science)
• 3body simulator – an example of a computer program that solves the three-body problem numerically

Notes and References

1. Encyclopedia: Barrow-Green. June. 2008. The Three-Body Problem. Gowers. Timothy. Barrow-Green. June. Leader. Imre. The Princeton Companion to Mathematics. 726–728. Princeton University Press.
2. Web site: Historical Notes: Three-Body Problem . 19 July 2017.
3. Book: Barrow-Green, June. Poincaré and the Three Body Problem. Poincaré and the Three-Body Problem . 1997. American Mathematical Society. 978-0-8218-0367-7. 8–12. 1997ptbp.book.....B.
4. Web site: The Three-Body Problem.
5. http://scienceworld.wolfram.com/physics/RestrictedThree-BodyProblem.html "Restricted Three-Body Problem"
6. Web site: The Three-Body Problem . . August 2019 . 7 May 2024.
7. An introduction to the classical three-body problem: From periodic solutions to instabilities and chaos . Krishnaswami . Govind S. . Senapati . Himalaya . Resonance . 24 . 87–114 . 2019 . Springer. 10.1007/s12045-019-0760-1 . 1901.07289 .
8. Jon . Cartwright . Physicists Discover a Whopping 13 New Solutions to Three-Body Problem . Science Now . 8 March 2013 . 2013-04-04.
9. Barrow-Green, J. (2010). The dramatic episode of Sundman, Historia Mathematica 37, pp. 164–203.
10. Beloriszky . D. . 1930 . Application pratique des méthodes de M. Sundman à un cas particulier du problème des trois corps . Bulletin Astronomique . 6 . Série 2 . 417–434. 1930BuAst...6..417B .
11. Burrau . Numerische Berechnung eines Spezialfalles des Dreikörperproblems. Astronomische Nachrichten . 195 . 6 . 113–118 . 1913 . 1913AN....195..113B . 10.1002/asna.19131950602.
12. Victor Szebehely . C. Frederick Peters . Complete Solution of a General Problem of Three Bodies . Astronomical Journal . 72 . 876 . 1967 . 10.1086/110355 . 1967AJ.....72..876S . free .
13. Web site: Šuvakov, M. . Dmitrašinović, V. . Three-body Gallery . 12 August 2015.
14. Moore . Cristopher . 1993PhRvL..70.3675M . 10.1103/PhysRevLett.70.3675 . 24 . Physical Review Letters . 3675–3679 . 10053934 . Braids in classical dynamics . 70 . 1993 . 2016-01-01 . 2018-10-08 . https://web.archive.org/web/20181008213647/http://tuvalu.santafe.edu/~moore/braids-prl.pdf .
15. Chenciner, Alain. Montgomery, Richard. A remarkable periodic solution of the three-body problem in the case of equal masses. Annals of Mathematics . Second Series. 152. 3. 2000. 881–902. 10.2307/2661357. math/0011268. 2661357. 2000math.....11268C. 10024592.
16. Montgomery . Richard . 48 . Notices of the American Mathematical Society . 471–481 . A new solution to the three-body problem . 2001.
17. Heggie . Douglas C. . 10.1046/j.1365-8711.2000.04027.x . 318 . 4 . Monthly Notices of the Royal Astronomical Society . L61–L63 . A new outcome of binary–binary scattering . 2000. free . astro-ph/9604016 . 2000MNRAS.318L..61H .
18. Hudomal . Ana . New periodic solutions to the three-body problem and gravitational waves . Master of Science Thesis at the Faculty of Physics, Belgrade University . October 2015 . 5 February 2019.
19. Li . Xiaoming . Liao . Shijun . More than six hundreds new families of Newtonian periodic planar collisionless three-body orbits . Science China Physics, Mechanics & Astronomy . December 2017 . 60 . 12 . 129511 . 10.1007/s11433-017-9078-5 . 1705.00527 . 1674-7348. 2017SCPMA..60l9511L . 84838204 .
20. Li . Xiaoming . Jing . Yipeng . Liao . Shijun . The 1223 new periodic orbits of planar three-body problem with unequal mass and zero angular momentum . Publications of the Astronomical Society of Japan . 70 . 4 . August 2018 . 64 . 1709.04775 . 10.1093/pasj/psy057 . free.
21. Li . Xiaoming . Liao . Shijun . Collisionless periodic orbits in the free-fall three-body problem . New Astronomy . 70 . 22–26 . 2019 . 1805.07980 . 10.1016/j.newast.2019.01.003 . 2019NewA...70...22L . 89615142 .
22. Three-body periodic collisionless equal-mass free-fall orbits revisited . Hristov . Ivan . Hristova . Radoslava . Dmitrašinović . Veljko . Tanikawa . Kiyotaka . Celestial Mechanics and Dynamical Astronomy . 2024 . 136 . 1 . 7 . 10.1007/s10569-023-10177-w . 2308.16159 . 2024CeMDA.136....7H .
23. Web site: 3body simulator . 2022-11-17 . 3body simulator . en . 2022-11-17 . https://web.archive.org/web/20221117041052/https://3body.hk/ . dead .
24. News: Technion . A Centuries-Old Physics Mystery? Solved . 12 October 2021 . SciTechDaily . . 6 October 2021.
25. Ginat . Yonadav Barry . Perets . Hagai B. . Analytical, Statistical Approximate Solution of Dissipative and Nondissipative Binary-Single Stellar Encounters . . 23 July 2021 . 11 . 3 . 031020 . 10.1103/PhysRevX.11.031020. 2011.00010 . 2021PhRvX..11c1020G . 235485570 . 12 October 2021.
26. Book: Valtonen, Mauri . The Three-body Problem from Pythagoras to Hawking . 2016 . Springer . 978-3-319-22726-9 . 1171227640.
27. Book: Newton, Isaac . Philosophiæ naturalis principia mathematica . London . G. & J. Innys . 1726 . 2022-10-05 . Hong Kong University of Science and Technology . 10.14711/spcol/b706487 .
28. Web site: Amerigo Vespucci . 2022-10-05 . Biography . 23 June 2021 . en-us.
29. The 1747 memoirs of both parties can be read in the volume of Histoires (including Mémoires) of the Académie Royale des Sciences for 1745 (belatedly published in Paris in 1749) (in French):

    Clairaut: "On the System of the World, according to the principles of Universal Gravitation" (at pp. 329–364); and

    d'Alembert: "General method for determining the orbits and the movements of all the planets, taking into account their mutual actions" (at pp. 365–390).The peculiar dating is explained by a note printed on page 390 of the "Memoirs" section: "Even though the preceding memoirs, of Messrs. Clairaut and d'Alembert, were only read during the course of 1747, it was judged appropriate to publish them in the volume for this year" (i.e. the volume otherwise dedicated to the proceedings of 1745, but published in 1749).

30. [Jean le Rond d'Alembert]
31. Mohr . R.F. . Furnstahl . R.J. . Hammer . H.-W. . Perry . R.J. . Wilson . K.G. . January 2006 . Precise numerical results for limit cycles in the quantum three-body problem . . 321 . 1 . 225–259 . 10.1016/j.aop.2005.10.002 . nucl-th/0509076 . 2006AnPhy.321..225M . 119073191 . 0003-4916.
32. https://babel.hathitrust.org/cgi/pt?id=uc1.32106020676935&view=1up&seq=122&skin=2021 "Coplanar Motion of Two Planets, One Having a Zero Mass"
33. Book: Barrow-Green, June . Poincaré and the Three Body Problem . 1996-10-29 . American Mathematical Society . 978-0-8218-0367-7 . History of Mathematics . 11 . Providence, Rhode Island. 10.1090/hmath/011 .
34. Efimov . V. . 1970-12-21 . Energy levels arising from resonant two-body forces in a three-body system . Physics Letters B . en . 33 . 8 . 563–564 . 10.1016/0370-2693(70)90349-7 . 1970PhLB...33..563E . 0370-2693.
35. Liao . Shijun . Li . Xiaoming . 2019-11-01 . On the periodic solutions of the three-body problem . National Science Review . en . 6 . 6 . 1070–1071 . 10.1093/nsr/nwz102 . 34691975 . 8291409 . 2095-5138.
36. Breen . Philip G. . Foley . Christopher N. . Boekholt . Tjarda . Portegies Zwart . Simon . Newton versus the machine: Solving the chaotic three-body problem using deep neural networks . 1910.07291 . 10.1093/mnras/staa713 . Monthly Notices of the Royal Astronomical Society . 494 . 2 . 2020 . 2465–2470 . free . 204734498.
37. News: Watson . Claire . We Just Got 12,000 New Solutions to The Infamous Three-Body Problem . 23 September 2023 . . live . https://archive.today/20230924005541/https://www.sciencealert.com/we-just-got-12000-new-solutions-to-the-infamous-three-body-problem . 24 September 2023 . 23 September 2023 .
38. Three-body periodic collisionless equal-mass free-fall orbits revisited . 2024 . 2308.16159 . Hristov . Ivan . Hristova . Radoslava . Dmitrašinović . Veljko . Tanikawa . Kiyotaka . Celestial Mechanics and Dynamical Astronomy . 136 . 1 . 10.1007/s10569-023-10177-w . 2024CeMDA.136....7H .
39. Book: Griffiths, David J.. Introduction to Quantum Mechanics (2nd ed.) . Prentice Hall . 2004 . 978-0-13-111892-8 . 40251748 . 311.
40. Crandall . R. . Whitnell . R. . Bettega . R. . Exactly soluble two-electron atomic model . American Journal of Physics . 52 . 5 . 438–442 . 1984 . 10.1119/1.13650 . 1984AmJPh..52..438C.
41. Calogero . F. . Solution of a Three-Body Problem in One Dimension . Journal of Mathematical Physics . 10 . 12 . 2191–2196 . 1969 . 10.1063/1.1664820 . 1969JMP....10.2191C.
42. Aref . Hassan . 1979-03-01 . Motion of three vortices . The Physics of Fluids . 22 . 3 . 393–400 . 10.1063/1.862605 . 1979PhFl...22..393A . 0031-9171.
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46. [Florin Diacu]