Variational Inference for SDEs
Driven by Fractional Noise

ICLR'24 (Spotlight)

1 D2LAB, Dept. Electromech. Systems and Metal Eng., Ghent University, Belgium, 2 Core lab EEDT Decision & Control, Flanders Make, Belgium, 3 Dept. of Theor. Comp. Science, Technical University of Berlin, Germany, 4 Inst. of Mathematics, University of Potsdam, Germany, 5 Centre for Systems Modelling and Quant. Biomed., University of Birmingham, UK, 6 Dept. of Computing, Imperial College London, UK
We present the first variational inference framework for non-Markovian neural SDEs driven by fractional Brownian Motion. Our method builds upon the idea of approximating the fBM by a linear combination of Markov processes, driven by the same, Brownian motion. We then provide the variational prior and posterior, as well as the EBLO.

Figure 1: We leverage the Markov approximation, where the non-Markovian fractional Brownian motion with Hurst index H is approximated by a linear combination of a finite number of Markov processes, and propose a variational inference framework in which the posterior is steered by a control term u(t). Note the long-term memory behaviour of the processes, where individual Y_k(t) have varying transient effects, from Y_1(t) having the longest memory to Y_7(t) the shortest, and tend to forget the action of u(t) after a certain time frame.

Abstract

We present a novel variational framework for performing inference in (neural) stochastic differential equations (SDEs) driven by Markov-approximate fractional Brownian motion (fBM). SDEs offer a versatile tool for modeling real-world continuous-time dynamic systems with inherent noise and randomness. Combining SDEs with the powerful inference capabilities of variational methods, enables the learning of representative function distributions through stochastic gradient descent. However, conventional SDEs typically assume the underlying noise to follow a Brownian motion (BM), which hinders their ability to capture long-term dependencies. In contrast, fractional Brownian motion (fBM) extends BM to encompass non-Markovian dynamics, but existing methods for inferring fBM parameters are either computationally demanding or statistically inefficient. In this paper, building upon the Markov approximation of fBM, we derive the evidence lower bound essential for efficient variational inference of posterior path measures, drawing from the well-established field of stochastic analysis. Additionally, we provide a closed-form expression to determine optimal approximation coefficients. Furthermore, we propose the use of neural networks to learn the drift, diffusion and control terms within our variational posterior, leading to the variational training of neural-SDEs. In this framework, we also optimize the Hurst index, governing the nature of our fractional noise. Beyond validation on synthetic data, we contribute a novel architecture for variational latent video prediction,—an approach that, to the best of our knowledge, enables the first variational neural-SDE application to video perception.

Application to Modeling Video as a Stochastic Process


Figure 4: Schematic of the latent SDE video model. Video frames {o_i} are encoded to vectors {h_i}. The static content vector w, that is free of the dynamic information, is inferred from {h_i}. The context model processes the information with temporal convolution layers, so that its outputs {g_i} contain information from neighbouring frames. A linear interpolation on {g_i} allows the posterior SDE model to receive time--appropriate information g(t), at (intermediate) time-steps chosen by the SDE solver. Finally, the states {x_i} and static w are decoded to reconstruct frames {o'_i}.


BibTeX

@inproceedings{daems2024variational,
title={Variational Inference for {SDE}s Driven by Fractional Noise},
author={Rembert Daems and Manfred Opper and Guillaume Crevecoeur and Tolga Birdal},
booktitle={The Twelfth International Conference on Learning Representations},
year={2024},
url={https://openreview.net/forum?id=rtx8B94JMS}
}

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