Nested Relation Extraction with Iterative Neural Network

Abstract

Natural language is used to describe objective facts, including simple relations like ""Jobs was the CEO of Apple"", and complex relations like ""the GDP of the United States in 2018 grew 2.9% compared with 2017". For the latter example, the growth rate relation is between two other relations. Due to the complex nature of language, this kind of nested relations is expressed frequently, especially in professional documents in fields like economics, finance, and biomedicine. But extracting nested relations is challenging, and research on this problem is almost vacant. In this paper, we formally formulate the nested relation extraction problem, and come up with a solution using Iterative Neural Network. Specifically, we observe that the nested relation structures can be expressed as a Directed Acyclic Graph (DAG), and propose the model to simultaneously consider the word sequence of natural language in the horizontal direction and the DAG structure in the vertical direction. Based on two nested relation extraction tasks, namely semantic causality relation extraction and formula extraction, we show that the proposed model works well on them. Moreover, we speed up the DAG-LSTM training significantly by a simple parallelization solution.