COVID-19 Literature Topic-Based Search via Hierarchical NMF. - Rachel Grotheer; Longxiu Huang; Yihuan Huang; Alona Kryshchenko; Oleksandr Kryshchenko; Pengyu Li; Xia Li; Elizaveta Rebrova; Kyung Ha; Deanna Needell (NLP4COVID@EMNLP, 2020)