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Inverse problem for Lagrangian mechanics : ウィキペディア英語版
Inverse problem for Lagrangian mechanics
In mathematics, the inverse problem for Lagrangian mechanics is the problem of determining whether a given system of ordinary differential equations can arise as the Euler–Lagrange equations for some Lagrangian function.
There has been a great deal of activity in the study of this problem since the early 20th century. A notable advance in this field was a 1941 paper by the American mathematician Jesse Douglas, in which he provided necessary and sufficient conditions for the problem to have a solution; these conditions are now known as the Helmholtz conditions, after the German physicist Hermann von Helmholtz.
==Background and statement of the problem==

The usual set-up of Lagrangian mechanics on ''n''-dimensional Euclidean space R''n'' is as follows. Consider a differentiable path ''u'' : () → R''n''. The action of the path ''u'', denoted ''S''(''u''), is given by
:S(u) = \int_^ L(t, u(t), \dot(t)) \, \mathrm t,
where ''L'' is a function of time, position and velocity known as the Lagrangian. The principle of least action states that, given an initial state ''x''0 and a final state ''x''1 in R''n'', the trajectory that the system determined by ''L'' will actually follow must be a minimizer of the action functional ''S'' satisfying the boundary conditions ''u''(0) = ''x''0, ''u''(T) = ''x''1. Furthermore, the critical points (and hence minimizers) of ''S'' must satisfy the Euler–Lagrange equations for ''S'':
:\frac t} \frac} - \frac 1 \leq i \leq n,
where the upper indices ''i'' denote the components of ''u'' = (''u''1, ..., ''u''''n'').
In the classical case
:T(\dot) = \frac m | \dot |^,
:V : (T ) \times \mathbb^ \to \mathbb,
:L(t, u, \dot) = T(\dot) - V(t, u),
the Euler–Lagrange equations are the second-order ordinary differential equations better known as Newton's laws of motion:
:\ddot^ = - \frac 1 \leq i \leq n,
:\mbox\ddot = - \nabla_ V(t, u).
The inverse problem of Lagrangian mechanics is as follows: given a system of second-order ordinary differential equations
:\ddot^ = f^ (u^, \dot^) \quad \text 1 \leq i, j \leq n, \quad \mbox
that holds for times 0 ≤ ''t'' ≤ ''T'', does there exist a Lagrangian ''L'' : () × R''n'' × R''n'' → R for which these ordinary differential equations (E) are the Euler–Lagrange equations? In general, this problem is posed not on Euclidean space R''n'', but on an ''n''-dimensional manifold ''M'', and the Lagrangian is a function ''L'' : () × T''M'' → R, where T''M'' denotes the tangent bundle of ''M''.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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