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| {{About|an ancient, classical problem in classical mechanics|the problem in quantum mechanics|Many-body problem}}
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| {{DISPLAYTITLE:''n''-body problem}}
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| In [[physics]], the '''''n''-body problem''' is an ancient, classical problem<ref name="Leimanis and Minorsky">Leimanis and Minorsky: Our interest is with Leimanis, who first discusses some history about the ''n''-body problem, especially Ms. Kovalevskaya's ~1868-1888, twenty-year complex-variables approach, failure; '''Section 1: The Dynamics of Rigid Bodies and Mathematical Exterior Ballistics''' (Chapter 1, ''the motion of a rigid body about a fixed point'' ('''Euler''' and '''Poisson''' ''equations''); Chapter 2, ''Mathematical Exterior Ballistics''), good precursor background to the ''n''-body problem; '''Section 2: Celestial Mechanics''' (Chapter 1, ''The Uniformization of the Three-body Problem'' (Restricted Three-body Problem); Chapter 2, ''Capture in the Three-Body Problem''; Chapter 3, ''Generalized n-body Problem'').</ref> of predicting the individual motions, and forces on same, of a group of [[astronomical object|celestial object]]s interacting with each other [[gravitation]]ally. Solving this problem — from the time of the Greeks and on — has been motivated by the desire to understand the motions of the [[Sun]], [[planet]]s and the visible [[star]]s. In the 20th century, understanding the dynamics of [[globular cluster]] star systems became an important ''n''-body problem too.<ref name="Heggie and Hut">See References sited for Heggie and Hut. This Wikipedia page has made their approach obsolete.</ref> The ''n''-body problem in [[general relativity]] is considerably more difficult to solve.
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| The classical physical problem can be informally stated as: ''given the quasi-steady orbital properties'' (''instantaneous position, velocity and time'')<ref>''Quasi-steady'' loads refers to the instantaneous inertial loads generated by instantaneous angular velocities and accelerations, as well as translational accelerations (9 variables). It is as though one took a photograph, which also recorded the instantaneous position and properties of motion. In contrast, a ''steady-state'' condition refers to a system's state being invariant to time; otherwise, the first derivatives and all higher derivatives are zero.</ref> ''of a group of celestial bodies, predict their interactive forces; and consequently, predict their true orbital motions for all future times''.<ref>R. M. Rosenberg states the ''n''-body problem similarly (see References): ''Each particle in a system of a finite number of particles is subjected to a Newtonian gravitational attraction from all the other particles, and to no other forces. If the initial state of the system is given, how will the particles move?'' Rosenberg failed to realize, like everyone else, that it is necessary to determine the forces ''first'' before the motions can be determined.</ref>
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| To this purpose the [[two-body problem]] has been completely solved and is discussed below; as is the famous ''Restricted 3-Body Problem''.<ref>A general, classical solution in terms of first integrals is known to be impossible. An exact theoretical solution for arbitrary ''n'' can be approximated via [[Taylor series]], but in practice such an [[infinite series]] must be truncated, giving at best only an approximate solution; and an approach now obsolete. In addition, the ''n''-body problem may be solved using [[numerical integration]], but these, too, are approximate solutions; and again obsolete. See Sverre J. Aarseth's book '''Gravitational N-body Simulations''' listed in the References.</ref>
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| This page next develops a general, closed-form ([[statics]]) solution for determining the reactive forces for the general ''n''-body problem case, owing to a concentrated applied body force (an external force and moment).
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| Rigid-body applications (i.e., where ''A'' 's = 1, see below) include distribution of useful loads in a [[Finite element method|finite element model]] (FEM); 3D rigid-body rivet analyses; and astronomy problems. [[Astrodynamics]] problems and the like are particular solutions that use this new, general ''n''-body problem solution as an algorithm; and once the forces of the bodies, {''m''<sub>ζ</sub>}, ζ = 1, 2, ... N, are known, velocities and accelerations, i.e. their motions, may be determined. Determining equations and numerical values for astronomy problems is beyond this page's scope. Structural analyses or soil analyses elastic solutions (''A'' 's < 1) are possible too. Again, the latter applications are beyond the scope of this page. Note especially: ''rigid-body solutions for elastic structures are not valid''.
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| An example problem is given.
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| ==Historical treatment==
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| Knowing three orbital positions of a planet's orbit – positions obtained by Sir [[Isaac Newton]] (1643-1727) from Astronomer [[John Flamsteed]]<ref>See David H. and Stephen P. H. Clark's '''The Suppressed Scientific Discoveries of Stephen Gray and John Flamsteed, Newton's Tyranny''', ''W. H. Freeman and Co''., 2001. A popularization of the historical events and bickering between those parties, but more importantly about the results they produced.</ref> – Newton was able to produce an equation by straightforward analytical geometry, to predict a planet's motion; i.e., to give its orbital properties: position, orbital diameter, period and orbital velocity.<ref>See "'''''Discovery of gravitation'', A.D. 1666'''" by [[Sir David Brewster]], in '''The Great Events by Famous Historians''', Rossiter Johnson, LL.D. Editor-in-Chief, Volume XII, pp. 51-65, ''The National Alumni'', 1905.</ref> Having done so he and others soon discovered over the course of a few years, those equations of motion did not predict very well or even correctly some orbits<ref>Rudolf Kurth has an extensive discussion in his book (see References) on planetary perturbations.</ref><sup>,</sup>;<ref>An aside: these mathematically undefined planetary perturbations (wobbles) still exist undefined even today and planetary orbits have to be constantly updated, usually yearly. See Astronomical Ephemeris and the American Ephemeris and Nautical Almanac, prepared jointly by the Nautical Almanae Offices of the United Kingdom and the United States of America.</ref> and Newton realized it was because gravitational interactive forces amongst all the planets was affecting all their orbits.
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| The above discovery goes right to the heart of the matter as to what exactly the ''n''-body problem is physically: as Newton realized, it is not sufficient to just specify the initial position and velocity, or three orbital positions either, to determine a planet's true orbit: ''the gravitational interactive forces have to be known too''. Thus came the awareness and rise of the n-body “problem” in the early 17th century. These gravitational attractive forces do conform to Newton's ''Laws of Motion'' and to his ''Law of Universal Gravitation'', but the many multiple (''n''-body) interactions have historically made any exact solution intractable. Ironically, this conformity led to the wrong approach.
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| After Newton's time the ''n''-body problem historically was not stated correctly ''because it did not include a reference to those gravitational interactive forces''. Newton does not say it directly but implies in his [[Philosophiæ Naturalis Principia Mathematica|Principia]] the ''n''-body problem is unsolvable because of those gravitational interactive forces.<ref>See '''Principia''', Book Three, ''System of the World'', "''General Scholium''," page 372, last paragraph. Newton was well aware his math model did not reflect physical reality. This edition referenced is from the '''Great Books of the Western World''', Volume 34, which was translated by Andrew Motte and revised by [[Florian Cajori]]. This same paragraph is on page 1160 in [[Stephen Hawkins]]' huge '''On the Shoulders of Giants''', 2002 edition; is a copy from Daniel Adee's 1848 addition. Cohen also has translated new editions: '''Introduction to Newton's 'Principia' ''', 1970; and '''Isaac Newton's ''Principia'', with Varian Readings''', 1972. Cajori also wrote a '''History of Science''', which is on the Internet.</ref> Newton said (ref. Cohen's '''Scientific American''' essay referenced below) in his '''Principia''', paragraph 21:
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| {{quote|And hence it is that the attractive force is found in both bodies. The Sun attracts Jupiter and the other planets, Jupiter attracts its satellites and similarly the satellites act on one another. And although the actions of each of a pair of planets on the other can be distinguished from each other and can be considered as two actions by which each attracts the other, yet inasmuch as they are between the same, two bodies they are not two but a simple operation between two termini. Two bodies can be drawn to each other by the contraction of rope between them. The cause of the action is twofold, namely the disposition of each of the two bodies; the action is likewise twofold, insofar as it is upon two bodies; but insofar as it is between two bodies it is single and one ...}}
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| Newton concluded via his 3rd Law that "according to this Law all bodies must attract each other." This last statement, which implies the existence of gravitational interactive forces, is key.
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| As shown below, the problem also conforms to [[Jean Le Rond D'Alembert]]'s non-Newtonian 1<sup>st</sup> and 2<sup>nd</sup> ''Principles'' and to the nonlinear ''n''-body problem algorithm, the latter allowing for a closed form solution for calculating those interactive forces.
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| ==Two-body problem==<!-- This section is linked from [[Conic section]] -->
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| {{Main|Two-body problem}}
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| Any discussion of planetary interactive forces has always started historically with the [[Two-body problem]]. The purpose of this Section is to relate the real complexity in calculating any planetary forces. Note in this Section also, several subjects, such as [[gravity]], [[barycenter]], [[Kepler's Laws]], etc.; and in the following Section too ([[Three-body problem]]) — are discussed on other Wilkipedia pages. Here though, these subjects are discussed from the perspective of the ''n''-body problem.
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| The two-body problem, ''n'' = 2, was completely solved by [[Johann Bernoulli]] (1667-1748) by ''classical'' theory (and not by Newton) by assuming the main point-mass was ''fixed'', is outlined here.<ref name="Bate, Mueller, and White">See Bate, Mueller, and White: Chapter '''1''', "'''''Two-Body Orbital Mechanics'''''," pp 1-49. These authors were from the ''Dept. of Astronautics and Computer Science'', United States Air Force Academy. See Chapter '''1'''. Their textbook is not filled with advanced mathematics.</ref> Consider then the motion of two bodies, say Sun-Earth, with the Sun ''fixed'', then:
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| <center>''m''<sub>1</sub>'''''a'''''<sub>1</sub> = (''Gm''<sub>1</sub>''m''<sub>2</sub>/''r''<sup>3</sup><sub>12</sub>)('''''r'''''<sub>2</sub> - '''''r'''''<sub>1</sub>)…..Sun-to-Earth</center>
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| <center>''m''<sub>2</sub>'''''a'''''<sub>2</sub> = (''Gm''<sub>2</sub>''m''<sub>1</sub>/''r''<sup>3</sup><sub>21</sub>)('''''r'''''<sub>1</sub> - '''''r'''''<sub>2</sub>)…..Earth-to-Sun</center>
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| The equation describing the motion of mass ''m''<sub>2</sub> relative to mass ''m''<sub>1</sub> is readily obtained from the differences between these two equations and after canceling common terms gives: ''α'' + (η/''r''<sup>3</sup>)'''''r''''' = 0, where
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| *''α'' is the ''Eulerian'' acceleration ''d''<sup>2</sup>'''''r'''''/''dt''<sup>2</sup>;
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| *'''''r''''' = '''''r'''''<sub>2</sub> - '''''r'''''<sub>1</sub> is the vector position of ''m''<sub>2</sub> relative to ''m''<sub>1</sub>;
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| *and η = ''G''(''m''<sub>1</sub> + ''m''<sub>2</sub>).
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| ''α'' + (η/''r''<sup>3</sup>)'''''r''''' = 0 is the fundamental differential equation for the two-body problem Bernoulli solved in 1734. Notice for this approach forces have to be determined first, then the equation of motion resolved. This differential equation has elliptic, or parabolic or hyperbolic solutions<ref>For the classical approach, if the common [[center of mass]] (i.e., the barycenter) of the two bodies is considered ''to be at rest'', then each body travels along a [[conic section]] which has a [[focus (geometry)|focus]] at the barycenter of the system. In the case of a hyperbola it has the branch at the side of that focus. The two conics will be in the same plane. The type of conic ([[circle]], [[ellipse]], [[parabola]] or [[hyperbola]]) is determined by finding the sum of the combined kinetic energy of two bodies and the [[potential energy#Gravitational potential energy|potential energy]] when the bodies are far apart. (This potential energy is always a negative value; energy of rotation of the bodies about their axes is not counted here)
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| *If the sum of the energies is negative, then they both trace out ellipses.
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| *If the sum of both energies is zero, then they both trace out parabolas. As the distance between the bodies tends to infinity, their relative speed tends to zero.
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| *If the sum of both energies is positive, then they both trace out hyperbolas. As the distance between the bodies tends to infinity, their relative speed tends to some positive number.</ref><sup>,</sup> <ref>For this approach see Lindsay's '''Physical Mechanics''', Chapter '''3''', "'''''Curvilinear Motion in a Plane'''''," and specifically paragraph ''3-9'', "'''''Planetary Motion'''''"; and continue reading on to the Chapter's end, pp. 83-96. Lindsay presentation goes a long way in explaining these latter comments for the fixed ''2-body problem''; i.e., when the Sun is assumed fixed.</ref><sup>,</sup>.<ref>Note: The fact a parabolic orbit has zero energy arises from the assumption the gravitational potential energy goes to zero as the bodies get infinitely far apart. One could assign ''any'' value to the potential energy in the state of infinite separation. That state is assumed to have zero potential energy ''by convention''.</ref>
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| It is incorrect to think of ''m''<sub>1</sub> (the Sun) as fixed in space when applying Newton's ''Law of Universal Gravitation'', and to do so leads to erroneous results. Dr. Clarence Cleminshaw calculated the approximate position of the Solar System's true barycenter, a result achieved mainly by combining only the masses of Jupiter and the Sun. ''' ''Science Program'' ''' stated in reference to his work: "''The Sun contains 98 per cent of the mass in the solar system, with the superior planets beyond Mars accounting for most of the rest. On the average, the center of the mass of the Sun-Jupiter system, when the two most massive objects are considered alone, lies 462,000 miles from the Sun's center, or some 30,000 miles above the solar surface! Other large planets also influence the center of mass of the solar system, however. In 1951, for example, the systems' center of mass was not far from the Sun's center because Jupiter was on the opposite side from Saturn, Uranus and Neptune. In the late 1950s, when all four of these planets were on the same side of the Sun, the system's center of mass was more than 330,000 miles form the solar surface, Dr. C. H. Cleminshaw of Griffith Observatory in Los Angeles has calculated''."<ref>'''Science Program''''s “''' ''The Nature of the Universe''' ''" states Clarence Cleminshaw (1902-1985) served as Assistant Director of ''Griffith Observatory'' from 1938-1958 and as Director from 1958-1969. Some publications by Cleminshaw, C. H.: “'''Celestial Speeds''',” 4 1953, equation, Kepler, orbit, comet, Saturn, Mars, velocity; Cleminshaw, C. H.: “'''The Coming Conjunction of Jupiter and Saturn''',” 7 1960, Saturn, Jupiter, observe, conjunction; Cleminshaw, C. H.: “'''The Scale of The Solar System''',” 7 1959, Solar system, scale, Jupiter, sun, size, light.</ref> This means the Sun wobbles and Sun-spots are possibly caused via the movement of the barycenter, owing to Jupiter's 11-year cycles, producing Sun-spots every 22 years. It further needs to be pointed out the total mass orbiting the Sun is probably equal to the Sun's own mass{{citation needed|date=January 2014}}.
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| [[File:Kipler's Error.jpg|thumbnail|The Real Motion v.s. Kepler's Apparent Motion]]
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| The Sun wobbles as it rotates around the galactic center, dragging the Solar System and Earth along with it. What mathematician [[Kepler]] did in arriving at his three famous equations was curve-fit the apparent motions of the planets using [[Tycho Brahe]]'s data, and ''not'' curve-fitting their true circular motions about the Sun (see Figure). Both [[Robert Hooke]] and Newton were well aware Newton's ''Law of Universal Gravitation'' did not hold for the forces associated with elliptical orbits.<ref>See. I. Bernard Cohen's '''''Scientific American''''' article.</ref> In fact, Newton's ''Universal Law'' doesn't account for the orbit of Mercury, the Asteroid Belt's gravitational behavior, or Saturn's Rings.<ref>Brush, Stephen G. Editor''':''' '''Maxwell on Saturn's Rings''', ''MIT Press'', 1983.</ref> Newton stated (in the 11th Section of the '''Principia''') the main reason however for failing to predict the forces for elliptical orbits was his math model was for a body confined to a situation that "''hardly exist in the real world''," namely, the "''motions of bodies attracted toward an unmoving center''." Some present physics and astronomy textbooks don't emphasize the negative significance of Newton's assumption and end up teaching that his math model is in effect reality. It is to be understood the classical two-body problem solution above is a mathematical toy. See also [[Kepler's laws of planetary motion#Kepler's first law|Kepler's first law of planetary motion]].<br />
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| Newton conveniently ''fixed'' the Sun so he could do simple calculations (in effect he cheated), but all following after him have also made the same mistake by analytically fixing the Sun too. They perpetuated the mathematical toy (see Truesdell's '''Essays in the History of Mechanics''' referenced below). An aside: ''Newtonian'' physics doesn't include (among other things) [[relative motion]] and may be the root of the reason Newton "''fixed''" the Sun.<ref>See [[Jacob Bronowski]] and Bruce Mazlish's '''The Western Intellectual Tradition''', ''Dorset Press'', 1986, for a discussion of this apparent lack of understanding by Newton. Also see Truesdell's '''Essays in the History of Mechanics''' for additional background about Newton accomplishments or lack therein.</ref><ref>As Hufbauer points out, Newton miscalculated and published unfortunately the wrong value for the Sun's mass twice before he got it correct in his third attempt.</ref> The Sun's wobbling means the ''n''-body problem's solution realistically is much more complicated than maybe previously thought (see below).
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| ==Three-body problem==<!-- This section is linked from [[Double planet]] -->
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| {{Main|Three-body problem}}
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| This Section relates an historically important ''n''-body problem solution after simplifying assumptions were made.
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| In the past not much was known about the ''n''-body problem for ''n'' equal to or greater than three.<ref>See Leimanis and Minorsky's historical comments.</ref> The case for ''n'' = 3 has been the most studied. Many earlier attempts to understand the ''Three-body problem'' were quantitative, aiming at finding explicit solutions for special situations.
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| *In 1687 [[Isaac Newton]] published in the '''Principia''' the first steps in the study of the problem of the movements of three bodies subject to their mutual gravitational attractions, but his efforts resulted in verbal descriptions and geometrical sketches; see especially Book 1, Proposition 66 and its corollaries (Newton, 1687 and 1999 (transl.), see also Tisserand, 1894).
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| *In 1767 [[Leonhard Euler|Euler]] found [[collinear]] motions, in which three bodies of any masses move proportionately along a fixed straight line. The [[Euler's three-body problem|circular restricted three-body problem]] is the special case in which two of the bodies are in circular orbits (approximated by the [[Sun]]-[[Earth]]-[[Moon]] system and many others).
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| *In 1772 [[Joseph Louis Lagrange|Lagrange]] discovered two classes of periodic solution, each for three bodies of any masses. In one class, the bodies lie on a rotating straight line. In the other class, the bodies lie at the vertices of a rotating equilateral triangle. In either case, the paths of the bodies will be conic sections. Those solutions led to the study of ''central configurations '', for which <math>\ddot q=kq</math> for some constant ''k>0 ''.
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| *A major study of the Earth-Moon-Sun system was undertaken by [[Charles-Eugène Delaunay]], who published two volumes on the topic, each of 900 pages in length, in 1860 and 1867. Among many other accomplishments, the work already hints at chaos, and clearly demonstrates the problem of so-called "''small denominators''" in [[perturbation theory]].
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| *In 1917 Forest Ray Moulton published his now classic, '''An Introduction to Celestial Mechanics''' (see references) with its plot of the ''Restricted Three-body Problem'' solution (see figure below).<ref>See Moulton's ''Restricted Three-body Problem'' 's analytical and graphical solution.</ref> An aside, see Meirovitch's book, pages 414 and 413 for his ''Restricted Three-body Problem'' solution.<ref>See Meirovitch's book: Chapters 11, ''Problems in Celestial Mechanics''; 12, ''Problem in Spacecraft Dynamics''; and Appendix A: ''Dyadics''.</ref>
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| [[Image:N-body problem (3).gif|frame|Motion of three particles under gravity, demonstrating [[chaos theory|chaotic]] behaviour]]
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| Moulton's solution may be easier to visualize (and definitely easier to solve) if one considers the more massive body (e.g., [[Sun]]) to be "''stationary''" in space, and the less massive body (e.g., [[Jupiter]]) to orbit around it, with the equilibrium points ([[Lagrangian point]]s) maintaining the 60 degree-spacing ahead of, and behind, the less massive body almost in its orbit (although in reality neither of the bodies are truly stationary, as they both orbit the center of mass of the whole system—about the barycenter). For sufficiently small mass ratio of the primaries, these triangular equilibrium points are stable, such that (nearly) massless particles will orbit about these points as they orbit around the larger primary (Sun). The five equilibrium points of the circular problem are known as the Lagrangian points. See figure below:
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| [[File:Restricted 3-Body 1.jpg|framed|center|Restricted 3-Body Problem]]
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| In the ''Restricted 3-Body Problem'' math model figure above (ref. Moulton), the Lagrangian points L<sub>4</sub> and L<sub>5</sub> are where the [[Trojan planet]]oids resided (see [[Lagrangian point]]); ''m''<sub>1</sub> is the Sun and ''m''<sub>2</sub> is Jupiter. L<sub>2</sub> is where the asteroid belt is. It has to be realized for this model, this whole Sun-Jupiter diagram is rotating about its barycenter. The ''Restricted 3-Body Problem'' solution predicted the Trojan planetoids before they were first seen. The ''h''-circles and closed loops echo the electromagnetic fluxes issued from the Sun and Jupiter. It is conjectured, contrary to Richard H. Batin conjecture (see References), the two h<sub>1</sub>'s are gravity sinks, in and where gravitational forces are zero, and the reason the Trojan planetoids are trap there. The total amount of mass of the planetoids is unknown.
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| The ''Restricted Three-body Problem'' assumes the [[mass]] of one of the bodies is negligible (???) {{Citation needed|date=September 2013}}. For a discussion of the case where the negligible body is a satellite of the body of lesser mass, see [[Hill sphere]]; for binary systems, see [[Roche lobe]]. Specific solutions to the ''Three-body problem'' result in [[chaos theory|chaotic]] motion with no obvious sign of a repetitious path.{{Citation needed|date=September 2013}}
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| The restricted problem (both circular and elliptical) was worked on extensively by many famous mathematicians and physicists, most notably by [[Henri Poincaré|Poincaré]] at the end of the 19th century. Poincaré's work on the ''Restricted Three-body Problem'' was the foundation of [[deterministic]] [[chaos theory]].{{Citation needed|date=September 2013}} In the restricted problem, there exist five [[equilibrium points]]. Three are collinear with the masses (in the rotating frame) and are unstable. The remaining two are located on the third vertex of both equilateral triangles of which the two bodies are the first and second vertices.
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| ==King Oscar II Prize: historical perspective==
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| The problem of finding the general solution of the ''n''-body problem was considered very important and challenging. Indeed in the late 19th century King [[Oscar II of Sweden]], advised by [[Gösta Mittag-Leffler]], established a prize for anyone who could find the solution to the problem. The announcement was quite specific:
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| <blockquote>Given a system of arbitrarily many mass points that attract each according to Newton's law, under the assumption that no two points ever collide, try to find a representation of the coordinates of each point as a series in a variable that is some known function of time and for all of whose values the series '''converges uniformly'''.</blockquote>
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| In case the problem could not be solved, any other important contribution to classical mechanics would then be considered to be prize-worthy.
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| The prize was finally awarded to Poincaré, even though he did not solve the original problem. (The first version of his contribution even contained a serious error; for details see the article by Diacu). The version finally printed contained many important ideas which led to the development of [[chaos theory]]. The problem as stated originally was finally solved by [[Karl F. Sundman|Karl Fritiof Sundman]] for ''n'' = 3 (see below).
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| == More general considerations for the classical solution ==
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| For every solution of the problem, not only applying an [[isometry]] or a time shift but also a [[T-symmetry|reversal of time]] (unlike in the case of friction) gives a solution as well.{{Citation needed|date=September 2013}}
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| In the physical literature about the <math>n</math>-body problem (<math>n</math> ≥ 3), sometimes reference is made to ''the impossibility of solving the <math>n</math>-body problem'' (via employing the above approach){{Citation needed|date=September 2013}}. However, care must be taken when discussing the 'impossibility' of a solution, as this refers only to the method of first integrals (compare the theorems by [[Niels Henrik Abel|Abel]] and [[Évariste Galois|Galois]] about the impossibility of solving [[Quintic equation|algebraic equations of degree five]] or higher by means of formulas only involving roots).
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| === (Classical) power series solution ===
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| This is the most elemental way classically of solving the ''n''-body problem. The theoretical expression is often called "The ''n''-body problem by [[Taylor series]]", which is an implementation of the [[Power series solution of differential equations]].
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| We start by defining the [[differential equations]] system{{Citation needed|date=September 2013}}:
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| <center><math>\underset{i}{\overset{2}{x_j}}(t)=G \sum _{k=0,k\neq i}^n \frac{\underset{k}{m} \left(\underset{k}{\overset{0}{x_j}}(t)-\underset{i}{\overset{0}{x_j}}(t)\right)}{\left(\left(\underset{k}{\overset{0}{x_1}}(t)-\underset{i}{\overset{0}{x_1}}(t)\right){}^2+\left(\underset{k}{\overset{0}{x_2}}(t)-\underset{i}{\overset{0}{x_2}}(t)\right){}^2+\left(\underset{k}{\overset{0}{x_3}}(t)-\underset{i}{\overset{0}{x_3}}(t)\right){}^2\right){}^{3/2}}</math>,</center>
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| where (in <math>\underset{i}{\overset{2}{x_j}}(t)</math>) the upper index <math>2</math> indicates the second derivative with respect to time <math>t</math>, <math>i</math> represents the number of each body and <math>j</math> the coordinate.
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| Because <math>\underset{i}{\overset{0}{x_j}}(t_0)</math> and <math>\underset{i}{\overset{1}{x_j}}(t_0)</math> are given as initial conditions, then every <math>\underset{i}{\overset{2}{x_j}}(t_0)</math> are known. Doing implicit derivation over every <math>\underset{i}{\overset{2}{x_j}}(t)</math> results in <math>\underset{i}{\overset{3}{x_j}}(t)</math> which at <math>t_0</math> are known because each depends on known <math>\underset{i}{\overset{k<3}{x_j}}(t_0)</math> precalculated and given constants and then the Taylor series are constructed theoretically in such way, performing this process infinitely.
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| ===Numerical integration===
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| {{main|N-body simulation}}
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| ''n''-body problems can be solved by numerically integrating the differential equations of motion. Many different ways to do this to varying degrees of accuracy and speed exist.<ref>[http://www.amara.com/papers/nbody.html N-Body/Particle Simulation Methods<!-- Bot generated title -->]</ref>
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| The simplest integrator is the [[Euler method]], but this is only first order. A second order method is [[leapfrog integration]], but higher-order integration methods such as the [[Runge–Kutta methods]] can be employed. [[Symplectic integrator]]s are often used for ''n''-body problems.
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| Numerical integration has a [[time complexity]] of {{math|''O''({{var|n}}{{sup|2}})}}, but tree structured methods, such as [[Barnes-Hut simulation]], can improve this to {{math|''O''({{var|n}} log {{var|n}})}}, or even to {{math|''O''({{var|n}})}} such as with the [[fast multipole method]].
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| ==Sundman's theorem for the 3-body problem==
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| In 1912, the [[Swedish-speaking Finns|Finnish]] mathematician [[Karl Fritiof Sundman]] proved there existed a series solution in powers of <math>t^{1/3}</math> for the ''3-body Problem''. This series is convergent for '''all ''' real ''t'', except initial data which correspond to ''zero angular momentum''. However these initial data are not generic since they have [[Lebesgue measure]] zero.
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| 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 ''3-body Problem''s. As it will be briefly discussed below, the only singularities in the ''3-body Problem'' are:
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| #binary collisions,
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| #triple collisions.
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| Now collisions, whether binary or triple (in fact of arbitrary order), are somehow improbable since it has been shown they correspond to a set of initial data of measure zero. However 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:
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| #He first was able, using an appropriate change of variables, to continue analytically the solution beyond the binary collision, in a process known as [[regularization (physics)|regularization]].
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| # He then proved triple collisions only occur when the angular momentum '''L''' vanishes. By restricting the initial data to <math>\mathbf{L}\neq 0</math> he removed all ''real '' singularities from the transformed equations for the 3-body problem.
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| # The next step consisted in showing that if <math>\mathbf{L}\neq 0</math>, then not only can there be no triple collision, but the system is strictly bounded away from a triple collision. This implies, by using [[Cauchy]]'s [[existence theorem]] for differential equations, there are no complex singularities in a strip (depending on the value of '''L''') in the complex plane centered around the real axis (shades of Kovalevskaya).
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| #The last step is then to find a conformal transformation which maps this strip into the unit disc. For example if <math>s=t^{1/3}</math> (the new variable after the regularization) and if <math>|\mathop{\text{Im}} \, s| \leq \beta</math> {{clarify|date=December 2009|reason=define terms }} then this map is given by:
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| <center><math>\sigma = \frac{e^{\pi s/(2\beta)} - 1}{e^{\pi s/(2\beta) }+1}.</math></center>
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| This finishes the proof of Sundman's ''Theorem''. Unfortunately the corresponding convergent series converges very slowly. That is, getting the value to any useful precision requires so many terms, that his solution is of little practical use.
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| ===A generalized Sundman global solution===
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| In order to generalize Sundman's result for the case ''n'' > 3 (or ''n'' = 3 and ''c'' = 0{{clarify | reason = what is ''c''?|date=July 2012}}) one has to face two obstacles:
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| #As it has been shown by Siegel, collisions which involve more than two bodies cannot be regularized analytically, hence Sundman's regularization cannot be generalized.
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| #The structure of singularities is more complicated in this case: other types of singularities may occur (see [[#Singularities of the n-body problem|below]]).
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| Lastly, Sundman's result was generalized to the case of ''n'' > 3 bodies by [[Qiudong Wang|Q. Wang]] in the 1990s. Since the structure of singularities is more complicated, Wang had to leave out completely the questions of singularities. The central point of his approach is to transform, in an appropriate manner, the equations to a new system, such that the interval of existence for the solutions of this new system is <math>[0,\infty)</math>.
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| ===Singularities of the ''n''-body problem===
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| There can be two types of singularities of the ''n''-body problem:
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| *collisions of two or more bodies, but for which '''''q'''''(''t'') (the bodies' positions) remains finite. (In this mathematical sense, a "collision" means that two point-like bodies have identical positions in space.)
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| *singularities in which a collision does not occur, but '''''q'''''(''t'') does not remain finite. In this scenario, bodies diverge to infinity in a finite time, while at the same time tending towards zero separation (an imaginary collision occurs "at infinity").
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| The latter ones are called Painlevé's conjecture (no-collisions singularities). Their existence has been conjectured for ''n'' > 3 by Painlevé (see Painlevé's conjecture). Examples of this behavior have been constructed by Xia<ref>{{cite journal |first=Zhihong |last=Xia |title=The Existence of Noncollision Singularities in Newtonian Systems |journal=[[Annals of Mathematics|Annals Math.]] |volume=135 |issue=3 |pages=411–468 |year=1992 |jstor=2946572 }}</ref> and Gerver.
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| ==See also==
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| *[[Einstein–Infeld–Hoffmann equations]]
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| *[[Few-body systems]]
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| *[[Natural units]]
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| *[[n-body choreography]]
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| *[[Virial theorem]]
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| {{subject bar|portal1=Mathematics|portal2=Physics|portal3=Astronomy}}
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| ==Notes==
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| {{reflist}}
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| ==References==
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| *Heggie, Douglas and Hut, Piet''': The Gravitational Million-Body Problem, A Multidisciplinary Approach to Star Cluster Dynamics''', ''Cambridge University Press'', 357 pages, 2003.
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| *Heggie, Douglas C.: "''Chaos in the N-body Problem of Stellar Dynamics''," in '''Predictability, Stability and Chaos in N-Body Dynamical Systems''', Ed. by Roy A. E., ''Plenum Press'', 1991.
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| *Aarseth, Sverre J.: '''Gravitational N-body Simulations, Tools and Algorithms''', ''Cambridge University Press'', 2003.
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| *van Winter, Clasine: "'''''The n-body problem on a Hilbert space of analytic functions'''''," Paper 11-29, in '''Analytic Methods in Mathematical Physics''', edited by Robert P. Gilbert and Roger G. Newton, pp. 569–578, ''Gordon and Breach'', 1970.
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| *Leimanis, E., and Minorsky, N.''': Dynamics and Nonlinear Mechanics''', Part '''I:''' ''Some Recent Advances in the Dynamics of Rigid Bodies and Celestial Mechanics'' (Leimanis), Part '''II:''' ''The Theory of Oscillations'' (Minorsky), ''John Wiley & Sons, Inc''., 1958.
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| *Moulton, Forest Ray: '''An Introduction to Celestial Mechanics''', ''Dover'', 1970.
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| *Kurth, Rudolf, '''Introduction to the Mechanics of the Solar System''', ''Pergamon Press'', 1959.
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| *Meirovitch, Leonard: '''Methods of Analytical Dynamics''', ''McGraw-Hill Book Co''., 1970.
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| *Brouwer, Dirk and Clemence, Gerald M.''': Methods of Celestial Mechanics''', ''Academic Press'', 1961.
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| *Cohen, Bernard I.: "'''''Newton's Discovery of Gravity'''''," '''''Scientific American''''', pp. 167–179, Vol. 244, No. 3, Mar. 1980.
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| *Cohen, Bernard I.: '''The Birth of a New Physics, Revised and Updated''', ''W.W. Norton & Co.'', 1985.
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| *''Science Program'''s “''' ''The Nature of the Universe'' ''',” booklet, published by ''Nelson Doubleday, Inc''., in 1968:
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| *Bate, Roger R.; Mueller, Donald D.; and White, Jerry''': Fundamentals of Astrodynamics''', ''Dover'', 1971.
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| *Battin, Richard H.''': An Introduction to The Mathematics and Methods of Astrodynamics''', '''''AIAA''''', 1987. He employs energy methods rather than a Newtonian approach.
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| *Gallian, Dave A. and Wilson, Henry E.: "'''''The Integration of NASTRAN Into Helicopter Airframe Design/Analysis'''''," '''''American Helicopter Society''''' Pub., Reprint No. 780, May 1973. This is a paper, not a book.
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| *Lindsay, Robert Bruce: '''Physical Mechanics''', 3rd Ed., ''D. Van Nostrand Co., Inc.'', 1961.
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| *Gelman, Harry: Part I: ''The second orthogonality conditions in the theory of proper and improper rotations: Derivation of the conditions and of their main consequences,'' J. Res. NBS 72B (Math. Sci.)No. 3, 1968. Part II: ''The intrinsic vector''; Part III: ''The Conjugacy Theorem'',J. Res. NBS 72B (Math. Sci.) No. 2, 1969. ''A Note on the time dependence of the effective axis and angle of a rotation,'' J. Res. NBS 72B (Math. Sci.)No. 3&4, Oct. 1971. These papers are on the Internet.
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| *Meriam, J. L.: '''Engineering Mechanics''', Volume 1 ''Statics'', Volume 2 ''Dynamics'', ''John Wiley & Sons'', 1978.
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| *Quadling, Henley: ''' "''Gravitational N-Body Simulation: 16 bit DOS version''," ''' June 1994. nbody*.zip is available at the http://www.ftp.cica.indiana.edu: see external links.
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| *Korenev, G. V.: '''The Mechanics of Guided Bodies''', '''''CRC''''' ''Press'', 1967.
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| *Eisele, John A. and Mason, Robert M.: '''Applied Matrix and Tensor Analysis''', ''John Wiley & Sons'', 1970.
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| *Murray, Carl D. and Dermott, Stanley F.: '''Solar System Dynamics''', ''Cambridge University Press'', 606 pages, 2000.
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| *Bronowski, Jacob and Mazlish, Bruce: '''The Western Intellectual Tradition, from Leonardo to Hegel''', ''Dorsey Press''. 1986.
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| *Truesdell, Clifford: '''Essays in the History of Mechanics''', ''Springer-Verlay'', 1968.
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| *Hufbauer, Dr. Karl C. (History of Science): '''Exploring the Sun, Solar Science since Galileo''', ''The Johns Hopkins University Press'', 1991. This book was sponsored by the '''NASA''' ''History Office''.
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| *Crandall, Richard E.: '''Topics in Advanced Scientific Computation''', Chapter 5, "''Nonlinear & Complex Systems''," paragraph 5.1, "''N-body problems & chaos''," pp. 215–221, ''Springer-Verlag'', 1996.
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| *Crandall, Richard E.: '''Projects in Scientific Computation''', Chapter 2, "''Exploratory Computation''," Project 2.4.1, "''Classical Physics''," pp. 93–97, corrected 3rd printing, ''Springer-Verlag'', 1996.
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| *Szebehely, Victor: '''Theory of Orbits''', ''Academic Press'', 1967.
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| *Rosenberg, Reinhardt M.: '''Analytical Dynamics, of Discrete Systems''', Chapter 19, ''About Celestial Problems'', paragraph 19.5, ''The n-body Problem'', pp. 364–371, ''Plenum Press'', 1977. Like Battin above, Rosenberg employs energy methods too, and to the solution of the general ''n''-body problem but doesn't actually solve anything.
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| *Diacu, F.: ''[http://www.math.uvic.ca/faculty/diacu/diacuNbody.pdf The solution of the n-body problem]'', The Mathematical Intelligencer,1996,18,p. 66–70
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| *Mittag-Leffler, G.: ''The n-body problem (Price Announcement)'', Acta Matematica, 1885/1886,7
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| *Saari, D.: ''A visit to the Newtonian n-body problem via Elementary Complex Variables'', ''American Mathematical Monthly'', 1990, 89, 105–119
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| *[[Isaac Newton|Newton, I.]]: ''Philosophiae Naturalis Principia Mathematica'', London, 1687: also English translation of 3rd (1726) edition by I. Bernard Cohen and Anne Whitman (Berkeley, CA, 1999).
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| *{{cite journal |authorlink=Qiudong Wang |last=Wang |first=Qiudong |bibcode=1991CeMDA..50...73W |title=The global solution of the n-body problem |journal=[[Celestial Mechanics and Dynamical Astronomy]] |issn=0923-2958 |volume=50 |issue=1 |year=1991 |pages=73–88 |accessdate=2007-05-05 |doi=10.1007/BF00048987}}
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| *Sundman, K. F.: ''Memoire sur le probleme de trois corps, Acta Mathematica 36 (1912): 105–179.
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| *Tisserand, F-F.: ''Mecanique Celeste'', tome III (Paris, 1894), ch.III, at p. 27.
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| *Hagihara, Y: Celestial Mechanics. (Vol I and Vol II pt 1 and Vol II pt 2.) MIT Press, 1970.
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| *Boccaletti, D. and Pucacco, G.: Theory of Orbits (two volumes). Springer-Verlag, 1998.
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| *Havel, Karel. N-Body Gravitational Problem: Unrestricted Solution (ISBN 978-09689120-5-8). Brampton: Grevyt Press, 2008. http://www.grevytpress.com
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| *{{cite journal |last=Saari |first=D. G. |last2=Hulkower |first2=N. D. |title=On the Manifolds of Total Collapse Orbits and of Completely Parabolic Orbits for the n-Body Problem |journal=Journal of Differential Equations |year=1981 |volume=41 |issue=1 |pages=27–43 |doi=10.1016/0022-0396(81)90051-6 }}
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| ==External links==
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| {{Div col|2}}
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| *[http://www.scholarpedia.org/article/Three_Body_Problem Three-Body Problem] at [[Scholarpedia]]
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| *[http://www.geom.umn.edu/~megraw/CR3BP_html/cr3bp.html More detailed information on the three-body problem]
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| *[http://www.ifmo.ru/butikov/ManyBody.pdf Regular Keplerian motions in classical many-body systems]
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| *[http://alecjacobson.com/programs/three-body-chaos/ Applet demonstrating chaos in restricted three-body problem]
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| *[http://www.ifmo.ru/butikov/Projects/Collection.html Applets demonstrating many different three-body motions]
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| *[http://www.datasync.com/~rsf1/manybod1.htm On the integration of the ''n''-body equations]
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| *[http://www.princeton.edu/~rvdb/JAVA/astro/galaxy/SolarSystem.html Java applet simulating Solar System]
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| *[http://www.princeton.edu/~rvdb/JAVA/astro/galaxy/StableRings.html Java applet simulating a ring of bodies orbiting a large central mass]
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| *[http://www.princeton.edu/~rvdb/JAVA/astro/galaxy/DustDisk.html Java applet simulating dust in the Solar System]
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| *[http://www.princeton.edu/~rvdb/JAVA/astro/galaxy/ThreeBody2.html Java applet simulating a stable solution to the equi-mass 3-body problem]
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| *[http://www.princeton.edu/~rvdb/JAVA/astro/galaxy/Galaxy0.html Java applet simulating choreographies and other interesting n-body solutions]
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| *[http://www.telefonica.net/web2/canrosin/index.htm A java applet to simulate the 3-d movement of set of particles under gravitational interaction]
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| *[http://orinetz.com/planet/viewsysblog.php?specific=QUQTS2CSDQ44FDURR3XD6NUD6 Javascript Simulation of our Solar System]
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| *[http://www.merlyn.demon.co.uk/gravity4.htm The Lagrange Points] - with links to the original papers of Euler and Lagrange, and to translations, with discussion
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| *http://ftp.math.utah.edu/pub/tex/bib/toc/sciam1980.html#242%281%29:January:1980
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| {{Div col end}}
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| {{orbits}}
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| {{DEFAULTSORT:N-Body Problem}}
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| [[Category:Gravitation]]
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| [[Category:Orbits]]
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| [[Category:Classical mechanics]]
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| [[Category:Computational problems]]
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