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Onsager–Machlup function : ウィキペディア英語版
Onsager–Machlup function
The Onsager–Machlup function is a function that summarizes the dynamics of a continuous stochastic process. It is used to define a probability density for a stochastic process, and it is similar to the Lagrangian of a dynamical system. It is named after Lars Onsager and S. Machlup who were the first to consider such probability densities.〔Onsager, L. and Machlup, S. (1953)〕
The dynamics of a continuous stochastic process from time to in one dimension, satisfying a stochastic differential equation
: dX_t = b(X_t)\,dt + \sigma(X_t)\,dW_t
where is a Wiener process, can in approximation be described by the probability density function of its value at a finite number of points in time :
: p(x_1,\ldots,x_n) = \left( \prod^_ \frac_ L\left(x_i,\frac\right) \, \Delta t_i \right)
where
: L(x,v) = \frac\left(\frac\right)^2
and , and . A similar approximation is possible for processes in higher dimensions. The approximation is more accurate for smaller time step sizes , but in the limit the probability density function becomes ill defined, one reason being that the product of terms
:\fract\in() \right)} \to \exp\left(-\int^T_0 L \left (\varphi_1(t),\dot_1(t) \right ) \, dt + \int^T_0 L \left (\varphi_2(t),\dot_2(t) \right) \, dt \right)
as , where is the Onsager–Machlup function.
==Definition==
Consider a -dimensional Riemannian manifold and a diffusion process on with infinitesimal generator , where is the Laplace–Beltrami operator and is a vector field. For any two smooth curves ,
:\lim_ \frac = \exp\left( -\int^T_0 L \left (\varphi_1(t),\dot_1(t) \right ) \, dt +\int^T_0 L \left (\varphi_2(t),\dot_2(t) \right ) \, dt \right)
where is the Riemannian distance, \scriptstyle \dot_1, \dot_2 denote the first derivatives of , and is called the Onsager–Machlup function.
The Onsager–Machlup function is given by〔Takahashi, Y. and Watanabe, S. (1980)〕〔Fujita, T. and Kotani, S. (1982)〕〔Wittich, Olaf〕
: L(x,v) = \tfrac\|v-b(x)\|_x^2 +\tfrac\operatorname\, b(x) - \tfracR(x),
where is the Riemannian norm in the tangent space at , is the divergence of at , and is the scalar curvature at .

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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