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Title: High-dimensional functional factor models Authors:  Shahin Tavakoli - University of Warwick (United Kingdom) [presenting]
Gilles Nisol - ULB (Belgium)
Marc Hallin - Universite Libre de Bruxelles (Belgium)
Abstract: Theoretical foundations are set up for high-dimensional approximate factor models for a panel of functional time series (FTS). We first establish a representation result stating that if the first r eigenvalues of the covariance operator of a cross-section of $N$ FTS are unbounded as $N$ diverges and if the $(r + 1)$th one is bounded, then we can represent each FTS as a sum of a common component driven by r factors, common to (almost) all the series, and a weakly cross-correlated idiosyncratic component (all the eigenvalues of the idiosyncratic covariance operator are bounded as $N$ diverges). The model and theory are developed in a general Hilbert space setting that allows for panels mixing functional and scalar time series. We then turn to the estimation of the factors, their loadings, and the common components. We derive consistency results in the asymptotic regime where the number $N$ of series and the number $T$ of time observations diverge, thus exemplifying the ``blessing of dimensionality'' that explains the success of factor models in the context of high-dimensional (scalar) time series. The results encompass the scalar case, for which they reproduce and extend, under weaker conditions, well-established results. We provide numerical illustrations and an empirical illustration on a dataset of intraday S\&P100 and Eurostoxx 50 stock returns, along with their scalar overnight returns.