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.

The dynamics of a continuous stochastic process X from time t = 0 to t = T in one dimension, satisfying a stochastic differential equation

$dX_{t}=b(X_{t})\,dt+\sigma (X_{t})\,dW_{t}$

where W is a Wiener process, can in approximation be described by the probability density function of its value x_i at a finite number of points in time t_i:

$p(x_{1},\ldots ,x_{n})=\left(\prod _{i=1}^{n-1}{\frac {1}{\sqrt {2\pi \sigma (x_{i})^{2}\Delta t_{i}}}}\right)\exp \left(-\sum _{i=1}^{n-1}L\left(x_{i},{\frac {x_{i+1}-x_{i}}{\Delta t_{i}}}\right)\,\Delta t_{i}\right)$

where

$L(x,v)={\frac {1}{2}}\left({\frac {v-b(x)}{\sigma }}\right)^{2}$

and Δt_i = t_i+1 − t_i > 0, t₁ = 0 and t_n = T. A similar approximation is possible for processes in higher dimensions. The approximation is more accurate for smaller time step sizes Δt_i, but in the limit where the time step sizes go to zero the probability density function becomes ill defined, one reason being that the product of terms $1/{\sqrt {2\pi \sigma (x_{i})^{2}\Delta t_{i}}}$ diverges to infinity. In order to nevertheless define a density for the continuous stochastic process X, ratios of probabilities of X lying within a small distance ε from smooth curves φ₁ and φ₂ are considered:

${\frac {P\left(|X_{t}-\phi _{1}(t)|\leq \varepsilon {\text{ for every }}t\in [0,T]\right)}{P\left(|X_{t}-\phi _{2}(t)|\leq \varepsilon {\text{ for every }}t\in [0,T]\right)}}$

converges to

$\exp \left(-\int _{0}^{T}L(\phi _{1}(t),{\dot {\phi }}_{1}(t))\,dt+\int _{0}^{T}L(\phi _{2}(t),{\dot {\phi }}_{2}(t))\,dt\right)$

as ε goes to zero, where L is the Onsager–Machlup function.

Definition

Consider a d-dimensional Riemannian manifold M and a diffusion process X = {X_t : 0 ≤ t ≤ T } on M with infinitesimal generator $\textstyle {\frac {1}{2}}$Δ_M + b, where Δ_M is the Laplace–Beltrami operator and b is a vector field. For any two smooth curves φ₁:[0,T] → M and φ₂:[0,T] → M,

${\begin{aligned}&\lim _{\varepsilon \downarrow 0}{\frac {P\left(\rho (X_{t},\phi _{1}(t))\leq \varepsilon {\text{ for every }}t\in [0,T]\right)}{P\left(\rho (X_{t},\phi _{2}(t))\leq \varepsilon {\text{ for every }}t\in [0,T]\right)}}\\&=\exp \left(-\int _{0}^{T}L(\phi _{1}(t),{\dot {\phi }}_{1}(t))\,dt+\int _{0}^{T}L(\phi _{2}(t),{\dot {\phi }}_{2}(t))\,dt\right)\end{aligned}}$

where ρ is the Riemannian distance, $\scriptstyle {\dot {\phi }}$₁ and $\scriptstyle {\dot {\phi }}$₂ denote the first derivatives of φ₁ and φ₂, and L is called the Onsager–Machlup function.

The Onsager–Machlup function is given by

$L(x,v)={\tfrac {1}{2}}\|v-b(x)\|_{x}^{2}+{\tfrac {1}{2}}\operatorname {div} \,b(x)-{\tfrac {1}{12}}R(x),$

where $\scriptstyle \|\,\cdot \,\|$_x is the Riemannian norm in the tangent space T_x(M) at x, div b(x) is the divergence of b at x, and R(x) is the scalar curvature at x.

Examples

The following examples give explicit expressions for the Onsager–Machlup function of a continuous stochastic processes.

Wiener process on the real line

The Onsager–Machlup function of a Wiener process on the real line R is given by

$L(x,v)={\tfrac {1}{2}}|v|^{2}.$

[Proof]

Let X = {X_t : 0 ≤ t ≤ T } be a Wiener process on R and let φ:[0,T] → R be a twice differentiable curve such that φ(0) = X₀.

Define another process X^φ = {X_t^φ : 0 ≤ t ≤ T } by X_t^φ = X_t − φ(t) and a measure P^φ by

$P^{\phi }=\exp \left(\int _{0}^{T}{\dot {\phi }}(t)\,dX_{t}^{\phi }+\int _{0}^{T}{\tfrac {1}{2}}|{\dot {\phi }}(t)|^{2}\,dt\right)\,dP.$

For every ε > 0, the probability that |X_t − φ_t| ≤ ε for every t ∈ [0,T] satisfies

${\begin{aligned}&P(|X_{t}-\phi (t)|\leq \varepsilon {\text{ for every }}t\in [0,T])\\[6pt]&=P(|X_{t}^{\phi }|\leq \varepsilon {\text{ for every }}t\in [0,T])\\[6pt]&=\int _{\{|X_{t}^{\phi }|\leq \varepsilon {\text{ for every }}t\in [0,T]\}}\exp \left(-\int _{0}^{T}{\dot {\phi }}(t)\,dX_{t}^{\phi }-\int _{0}^{T}{\tfrac {1}{2}}|{\dot {\phi }}(t)|^{2}\,dt\right)\,dP^{\phi }.\end{aligned}}$

By Girsanov's theorem, the distribution of X^φ under P^φ equals the distribution of X under P, hence the latter can be substituted by the former:

${\begin{aligned}&P(|X_{t}-\phi (t)|\leq \varepsilon {\text{ for every }}t\in [0,T])\\&=\int _{\{|X_{t}|\leq \varepsilon {\text{ for every }}t\in [0,T]\}}\exp \left(-\int _{0}^{T}{\dot {\phi }}(t)\,dX_{t}-\int _{0}^{T}{\tfrac {1}{2}}|{\dot {\phi }}(t)|^{2}\,dt\right)\,dP.\end{aligned}}$

By Itō's lemma it holds that

$\int _{0}^{T}{\dot {\phi }}(t)\,dX_{t}={\dot {\phi }}(T)X_{T}-\int _{0}^{T}{\ddot {\phi }}(t)X_{t}\,dt,$

where $\scriptstyle {\ddot {\phi }}$ is the second derivative of φ, and so this term is of order ε on the event where |X_t| ≤ ε for every t ∈ [0, T] and will disappear in the limit where ε goes to zero, hence

$\lim _{\varepsilon \downarrow 0}{\frac {P(|X_{t}-\phi (t)|\leq \varepsilon {\text{ for every }}t\in [0,T])}{P(|X_{t}|\leq \varepsilon {\text{ for every }}t\in [0,T])}}=\exp \left(-\int _{0}^{T}{\tfrac {1}{2}}|{\dot {\phi }}(t)|^{2}\,dt\right).$

Diffusion processes with constant diffusion coefficient on Euclidean space

The Onsager–Machlup function in the one-dimensional case with constant diffusion coefficient σ is given by

$L(x,v)={\frac {1}{2}}\left|{\frac {v-b(x)}{\sigma }}\right|^{2}+{\frac {1}{2}}{\frac {db}{dx}}(x).$

In the d-dimensional case, with σ equal to the unit matrix, it is given by

$L(x,v)={\frac {1}{2}}\|v-b(x)\|^{2}+{\frac {1}{2}}(\operatorname {div} \,b)(x),$

where $\scriptstyle \|\,\cdot \,\|$ is the Euclidean norm and (div b)(x) = ∑^d_i=1 $\textstyle {\frac {1}{2}}{\frac {\partial }{\partial x_{i}}}$ b_i(x) .

Generalizations

Generalizations have been obtained by weakening the differentiability condition on the curve φ. Rather than taking the maximum distance between the stochastic process and the curve over a time interval, other conditions have been considered such as distances based on completely convex norms and Hölder, Besov and Sobolev type norms.

Applications

The Onsager–Machlup function can be used for purposes of reweighting and sampling trajectories, as well as for determining the most probable trajectory of a diffusion process.

See also

• Lagrangian
• Functional integration

Bibliography

• Adib, A.B. (2008). "Stochastic actions for diffusive dynamics: Reweighting, sampling, and minimization". J. Phys. Chem. B. 112: 5910–5916. doi:10.1021/jp0751458.
• Capitaine, M. (1995). "Onsager–Machlup functional for some smooth norms on Wiener space". Probab. Theory Relat. Fields. 102: 189–201. doi:10.1007/bf01213388.
• Dürr, D. and Bach, A. (1978). "The Onsager–Machlup function as Lagrangian for the most probable path of a diffusion process". Commun. Math. Phys. 60: 153–170. doi:10.1007/bf01609446.{{cite journal}}: CS1 maint: multiple names: authors list (link)
• Fujita, T. and Kotani, S. (1982). "The Onsager–Machlup function for diffusion processes". J. Math. Kyoto Univ. 22: 115–130.{{cite journal}}: CS1 maint: multiple names: authors list (link)
• Ikeda, N. and Watanabe, S. (1980). Stochastic differential equations and diffusion processes. Kodansha-John Wiley.{{cite book}}: CS1 maint: multiple names: authors list (link)
• Onsager, L. and Machlup, S. (1953). "Fluctuations and Irreversible Processes". Physical Review. 91 (6): 1505–1512. doi:10.1103/physrev.91.1505.{{cite journal}}: CS1 maint: multiple names: authors list (link)
• Shepp, L. and Zeitouni, O. (1993). "Exponential estimates for convex norms and some applications". Progress in Probability. 32. Berlin. Birkhauser-Verlag: 203–215. doi:10.1007/978-3-0348-8555-3_11.{{cite journal}}: CS1 maint: multiple names: authors list (link)
• Stratonovich, R. (1971). "On the probability functional of diffusion processes". Select. Transl. in Math. Stat. Prob. 10: 273–286.
• Takahashi, Y. and Watanabe, S. (1980). "The probability functionals (Onsager–Machlup functions) of diffusion processes". Springer Lecture Notes in Math. 851: 432–463.{{cite journal}}: CS1 maint: multiple names: authors list (link)
• Wittich, Olaf. "The Onsager–Machlup Functional Revisited". {{cite journal}}: Cite journal requires |journal= (help)
• Zeitouni, O. (1989). "On the Onsager–Machlup functional of diffusion processes around non C² curves". Annals of Probability. 17 (3): 1037–1054. doi:10.1214/aop/1176991255.

External links

• Onsager–Machlup function. Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Onsager-Machlup_function&oldid=22857