1 INTRODUCTION

2 RELATED WORK  
2.1 Pre-trained Language Models  
2.2 Knowledge Representation

3 CLASSIFICATION OF KNOWLEDGE ENHANCED PRE-TRAINED LANGUAGE MODELS  
3.1 The Principle of Classification  
3.2 A Taxonomy of Knowledge Enhanced Pre-trained Language Models

4 OVERVIEW OF KNOWLEDGE ENHANCED PRE-TRAINED LANGUAGE MODELS

5 CONCLUSION AND FUTURE DIRECTIONS