Title: Recurrent Neural Networks are Universal Filters. Abstract: Recurrent neural networks (RNN) are powerful time series modeling tools in ma-
chine learning. It has been successfully applied in a variety of fields such as natural
language processing (Mikolov et al. (2010), Graves et al. (2013), Du et al. (2015)),
control (Fei & Lu (2017)) and traffic forecasting (Ma et al. (2015)), etc. In those
application scenarios, RNN can be viewed as implicitly modelling a stochastic dy-
namic system. Another type of popular neural network, deep (feed-forward) neural
network has also been successfully applied in different engineering disciplines,
whose approximation capability has been well characterized by universal approxi-
mation theorem (Hornik et al. (1989), Park & Sandberg (1991), Lu et al. (2017)).
However, the underlying approximation capability of RNN has not been fully
understood in a quantitative way. In our paper, we consider a stochastic dynamic
system with noisy observations and analyze the approximation capability of RNN
in synthesizing the optimal state estimator, namely optimal filter. We unify the
recurrent neural network into Bayesian filtering framework and show that recurrent
neural network is a universal approximator of optimal finite dimensional filters
under some mild conditions. That is to say, for any stochastic dynamic systems
with noisy sequential observations that satisfy some mild conditions, we show that
(informal)
∀ > 0, ∃ RNN-based filter, s.t. lim sup x̂ k|k − E[x k |Y k ] < ,
k→∞
where x̂ k|k is RNN-based filter’s estimate of state x k at step k conditioned on
the observation history and E[x k |Y k ] is the conditional mean of x k , known as the
optimal estimate of the state in minimum mean square error sense. As an interesting
special case, the widely used Kalman filter (KF) can be synthesized by RNN.