  Processing sequential data of variable length is a major challenge in a wide
range of applications, such as speech recognition, language modeling,
generative image modeling and machine translation. Here, we address this
challenge by proposing a novel recurrent neural network (RNN) architecture, the
Fast-Slow RNN (FS-RNN). The FS-RNN incorporates the strengths of both
multiscale RNNs and deep transition RNNs as it processes sequential data on
different timescales and learns complex transition functions from one time step
to the next. We evaluate the FS-RNN on two character level language modeling
data sets, Penn Treebank and Hutter Prize Wikipedia, where we improve state of
the art results to $1.19$ and $1.25$ bits-per-character (BPC), respectively. In
addition, an ensemble of two FS-RNNs achieves $1.20$ BPC on Hutter Prize
Wikipedia outperforming the best known compression algorithm with respect to
the BPC measure. We also present an empirical investigation of the learning and
network dynamics of the FS-RNN, which explains the improved performance
compared to other RNN architectures. Our approach is general as any kind of RNN
cell is a possible building block for the FS-RNN architecture, and thus can be
flexibly applied to different tasks.
