TY - JOUR T1 - Preface: Special Issue Dedicated to Professor Roger Penrose's 90th Birthday AU - JO - Communications in Mathematical Research VL - 4 SP - i EP - ii PY - 2021 DA - 2021/08 SN - 37 DO - http://doi.org/10.4208/cmr.2021.v37n4.preface UR - https://global-sci.org/intro/article_detail/cmr/19436.html KW - AB -