We mourn the loss of Richard Kenneth Guy who passed away on March 9, 2020, at the impressive age of 103. Richard was a mathematical giant who made enormous and lasting contributions to our discipline. Active till the end, he was a distinguished researcher, a passionate educator, a generous philanthropist and an avid mountaineer. To us, he was also a valued colleague, mentor and friend.

This collection of contributions from colleagues and former students at the University of Calgary recounts some of Richard’s involvement in research and mentorship, with special emphasis on the more than 13 years of his life after ninety. Richard’s interests were too diverse and his contributions too numerous to do them all justice in one article. As a result, we chose to focus on three main areas of mathematics, drawing from experts in these fields for their recollections and perspectives on Richard’s impact. A section on combinatorial game theory was provided by Richard’s former doctoral student Richard Nowakowski. Richard’s University of Calgary colleague Tibor (Ted) Bisztriczky supplied a section on geometry. A section on number theory was written by Richard’s Calgary colleagues Mike Jacobson, Hugh Williams and the first author. Outreach and mentorship were equally important to Richard as research, and he has inspired many young scholars. We solicited input from two of his former University of Calgary protégés, Alex Fink and Julian Salazar, with whom he kept in contact until the last weeks of his life. We have also included a brief itemized biography and a short epilogue offering insight into other aspects of Richard’s life both in and outside mathematics.

We thank all the contributors and are indebted to Claude Levesque (Université Laval) for providing the French translation of the English original. The photographs herein are courtesy of the University of Calgary, Ted Bisztriczky, Yanmei Fei, Jane Lancaster, Chic Scott, Hugh Williams and the first author.