In the Presence of Greatness -- A Few Abiding Memories of Dijkstra's Class

by Xiaozhou Li

At the Memorial Celebration in Honor of E. W. Dijkstra
November 21, 2002

At 9 o'clock on a fine Tuesday morning, September 1997, my home phone rang. As a hard-working graduate student, I was fast asleep at that time. As soon as I picked up the phone, I was totally awake: Dijkstra was calling. I surely knew who Dijkstra was, I just didn't have a clue why he called. No, I actually had a clue: A couple of days before, I had left a note at the door to his office, expressing my interest in sitting in his class, one that was offered to undergraduate students. I never thought that he would call. After dismissing a sequence of apologies from me, he patiently explained to me that the class was full and he was planning to offer a class to graduate students the next Fall, and I could attend that class if I wished.

I sure did. When the next Fall came, the opportunity was not to be missed. A year before, during an advising meeting to new graduate students, Prof. Porter, then the graduate advisor, had warned the eager young minds that Dijkstra's course would be life-altering. He was right. Five minutes into the first lecture, I learned something that I shall never forget, and that is, ideas are vague until you have written them down, so write down your proofs before presenting them to others. Also unforgettable was his insistence on writing proofs starting from the very upper-left corner of the blackboard. Applying this technique to paper, I am amazed how many times I have been able to keep my writing to one natural unit: a page, not to mention the amount of scratch paper henceforth saved.

Besides intriguing problems and elegant proofs, Dijkstra's class was full of wonders. From meticulous symbol manipulation precise down to every single punctuation, to the clarity, precision, and conciseness of his words, e.g., "at most" instead of "less than or equal to", "at least" instead of "greater than or equal to". Of course, his class was in constant touch with world news and general purpose research topics, e.g., the event that 25 journalists died in 26 countries, panchromatic concept animation based on 3-D artificial intuition nets, and distributed conflict resolution in the presence of non-monotonic belief systems.

A binder on my bookshelf keeps the lecture notes of his class and the photo of me taken by him at the beginning of one of his lectures, with his signature at the back. That photo, although not my favorite, accurately records my tidiness, or lack of it, at 9:30 that morning. From time to time, I leaf through the lecture notes, and when I come to the last example he showed in his class, an elegant proof of Fermat's Little Theorem, I always remember what he said after showing that example, a unique colloquial expression at a unique moment, "Ain't it a beauty!"

Thank you.