\[\renewcommand{\var}{\operatorname{Var}} \renewcommand{\dd}{\mathrm{d}} \renewcommand{\bb}[1]{\mathbb{#1}} \renewcommand{\vv}[1]{\boldsymbol{#1}} \renewcommand{\rv}[1]{\mathsf{#1}} \renewcommand{\gvn}{\mid} \renewcommand{\Ex}{\mathbb{E}} \renewcommand{\Pr}{\mathbb{P}}\]

“When are the paths of a stochastic process continuous?” is a question one might like to ask. But we need to ask more precise questions than that because this is complicated in probability land. If we are concerned about whether the paths sampled from the process are almost-surely continuous functions then we probably mean something like:

“Does the process \(\{\rv{x}(t)\}_t\) admit a modification such that \(t\mapsto \rv{x}(t)\) is in Hölder-continuous with probability 1?”

More generally, there are many notions of continuity of stochastic processes.

## Kolmogorov continuity theorem

The Kolmogorov continuity theorem gives us sufficient conditions for Hölder-path-continuity of the process based on how rapidly moments of the process increments grow. Question: What gives us sufficient conditions? Lowther is good on this.

## Stochastic continuity

An entirely different thing. Feller-continuity etc.

## Continuity entailments

🏗

## SDEs with rough paths

## “Random DEs”

Smooth SDEs. (Bongers and Mooij 2018)

## Connection to weak solutions of SDEs

TBD.

## Continuity of Gaussian processes

Todo: Read Kanagawa et al. (2018) section 4, which has some startling revelations:

… it is easy to show that a GP sample path \(\mathrm{f} \sim \mathcal{G P}(0, k)\) does not belong to the corresponding RKHS \(\mathcal{H}_{k}\) with probability 1 if \(\mathcal{H}_{k}\) is infinite dimensional, as summarized in Corollary \(4.10\) below. This implies that GP samples are “rougher”, or less regular, than RKHS functions (see also Figure 2). Note that this fact has been well known in the literature; see e.g., (Wahba 1990, 5) and (Lukić and Beder 2001 Corollary 7.1).

Let \(k\) be a positive definite kernel on a set \(\mathcal{X}\) and \(\mathcal{H}_{k}\) be its RKHS, and consider \(\mathrm{f} \sim \mathcal{G} \mathcal{P}(m, k)\) with \(m: \mathcal{X} \rightarrow \mathbb{R}\) satisfying \(m \in \mathcal{H}_{k} .\) Then if \(\mathcal{H}_{k}\) is infinite dimensional, then \(\mathrm{f} \in \mathcal{H}_{k}\) with probability \(0 .\) If \(\mathcal{H}_{k}\) is finite dimensional, then there is a version \(\tilde{\mathrm{f}}\) of \(\mathrm{f}\) such that \(\tilde{\mathrm{f}} \in \mathcal{H}_{k}\) with probability \(1 .\)

## References

*Transactions of the American Mathematical Society*353 (10): 3945–69. https://doi.org/10.1090/S0002-9947-01-02852-5.

*Revista Matemática Iberoamericana*14 (2): 215–310. https://doi.org/10.4171/RMI/240.

*Differential Equations Driven by Rough Paths*. Vol. 1908. Lecture Notes in Mathematics. Springer, Berlin. https://mathscinet.ams.org/mathscinet-getitem?mr=2314753.

*Spline Models for Observational Data*. SIAM.

## No comments yet!