Density-valued time series: Nonparametric density-on-density regression

This paper is concerned with forecasting probability density functions. Density functions are nonnegative and have a constrained integral; they thus do not constitute a vector space. Implementing unconstrained functional time-series forecasting methods is problematic for such nonlinear and constrained data. A novel forecasting method is developed based on a nonparametric function-on-function regression, where both the response and the predictor are probability density functions. Through a series of Monte-Carlo simulation studies, we evaluate the finite-sample performance of our nonparametric regression estimator. Using French departmental COVID19 data and age-specific period life tables in the United States, we assess and compare finite-sample forecast accuracy between the proposed and several existing methods.