Exponential mixing properties of nonlinear functional autoregressive models

The importance of functional data analysis has increased substantially in recent years. In machine learning, nonlinear function regression based on deep neural networks is referred to as operator learning, and many of its applications involve functional time series data. However, the theoretical understanding of nonlinear models in functional time series analysis remains limited, as most existing works focus on linear models. In this paper, we derive basic properties for analyzing adaptive learning in nonlinear functional autoregressive (NFAR) models. Specifically, we derive sufficient conditions for NFAR models to be exponentially mixing. We provide an example with a Hammerstein operator under which these conditions are satisfied. As an application of exponential mixing, we consider operator learning for NFAR models with Urysohn operators and derive convergence rates for adaptive estimators based on deep neural networks.