Fundamentals of Recurrent Neural Network (RNN) and Long Short-Term Memory (LSTM) network
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TL;DR
A Machine Learning practitioner seeking guidance for implementing the new augmented LSTM model in software for experimentation and research will find the insights and derivations in this treatise valuable as well.
Abstract
Because of their effectiveness in broad practical applications, LSTM networks\nhave received a wealth of coverage in scientific journals, technical blogs, and\nimplementation guides. However, in most articles, the inference formulas for\nthe LSTM network and its parent, RNN, are stated axiomatically, while the\ntraining formulas are omitted altogether. In addition, the technique of\n"unrolling" an RNN is routinely presented without justification throughout the\nliterature. The goal of this paper is to explain the essential RNN and LSTM\nfundamentals in a single document. Drawing from concepts in signal processing,\nwe formally derive the canonical RNN formulation from differential equations.\nWe then propose and prove a precise statement, which yields the RNN unrolling\ntechnique. We also review the difficulties with training the standard RNN and\naddress them by transforming the RNN into the "Vanilla LSTM" network through a\nseries of logical arguments. We provide all equations pertaining to the LSTM\nsystem together with detailed descriptions of its constituent entities. Albeit\nunconventional, our choice of notation and the method for presenting the LSTM\nsystem emphasizes ease of understanding. As part of the analysis, we identify\nnew opportunities to enrich the LSTM system and incorporate these extensions\ninto the Vanilla LSTM network, producing the most general LSTM variant to date.\nThe target reader has already been exposed to RNNs and LSTM networks through\nnumerous available resources and is open to an alternative pedagogical\napproach. A Machine Learning practitioner seeking guidance for implementing our\nnew augmented LSTM model in software for experimentation and research will find\nthe insights and derivations in this tutorial valuable as well.\n
