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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 : 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'': : where the upper indices ''i'' denote the components of ''u'' = (''u''1, ..., ''u''''n''). In the classical case : : : the Euler–Lagrange equations are the second-order ordinary differential equations better known as Newton's laws of motion: : : The inverse problem of Lagrangian mechanics is as follows: given a system of second-order ordinary differential equations : 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)』 ■ウィキペディアで「Inverse problem for Lagrangian mechanics」の詳細全文を読む スポンサード リンク
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