Deep Residual Echo State Networks: exploring residual orthogonal
  connections in untrained Recurrent Neural Networks
Authors: name: Matteo Pinna, name: Andrea Ceni, name: Claudio Gallicchio

Echo State Networks (ESNs) are a particular type of untrained Recurrent
Neural Networks (RNNs) within the Reservoir Computing (RC) framework, popular
for their fast and efficient learning. However, traditional ESNs often struggle
with long-term information processing. In this paper, we introduce a novel
class of deep untrained RNNs based on temporal residual connections, called
Deep Residual Echo State Networks (DeepResESNs). We show that leveraging a
hierarchy of untrained residual recurrent layers significantly boosts memory
capacity and long-term temporal modeling. For the temporal residual
connections, we consider different orthogonal configurations, including
randomly generated and fixed-structure configurations, and we study their
effect on network dynamics. A thorough mathematical analysis outlines necessary
and sufficient conditions to ensure stable dynamics within DeepResESN. Our
experiments on a variety of time series tasks showcase the advantages of the
proposed approach over traditional shallow and deep RC.