Markov Processes, Hurst Exponents, and Nonlinear Diffusion Equations with application to finance

TL;DR

This paper demonstrates that Markov processes can have Hurst exponents different from 1/2 without implying long-term correlations, challenging common assumptions in stochastic process analysis and applying this to finance models.

Contribution

It provides explicit solutions showing Markov processes can exhibit non-1/2 Hurst exponents without long-term correlations, with applications to nonlinear diffusion and finance.

Findings

Markov processes can have Hurst exponent H ≠ 1/2 without long-term correlations.

Tsallis density can be derived from a linear Fokker-Planck equation, not nonlinear diffusion.

Hurst exponent alone does not indicate the presence of correlations in Markov processes.

Abstract

We show by explicit closed form calculations that a Hurst exponent H that is not 1/2 does not necessarily imply long time correlations like those found in fractional Brownian motion. We construct a large set of scaling solutions of Fokker-Planck partial differential equations where H is not 1/2. Thus Markov processes, which by construction have no long time correlations, can have H not equal to 1/2. If a Markov process scales with Hurst exponent H then it simply means that the process has nonstationary increments. For the scaling solutions, we show how to reduce the calculation of the probability density to a single integration once the diffusion coefficient D(x,t) is specified. As an example, we generate a class of student-t-like densities from the class of quadratic diffusion coefficients. Notably, the Tsallis density is one member of that large class. The Tsallis density is usually thought to result from a nonlinear diffusion equation, but instead we explicitly show that it follows from a Markov process generated by a linear Fokker-Planck equation, and therefore from a corresponding Langevin equation. Having a Tsallis density with H not equal to 1/2 therefore does not imply dynamics with correlated signals, e.g., like those of fractional Brownian motion. A short review of the requirements for fractional Brownian motion is given for clarity, and we explain why the usual simple argument that H unequal to 1/2 implies correlations fails for Markov processes with scaling solutions. Finally, we discuss the question of scaling of the full Green function g(x,t;x',t') of the Fokker-Planck pde.