Monday, October 11, 2010
Alf van der Poorten (1942-2010)
I just received the sad news that my friend and colleague, Alf van der Poorten, passed away on Saturday in Australia.
Alf (or "vdp", as he was sometimes called) specialized in number theory, and he wrote about 170 papers on the subject, specializing in continued fractions, elliptic curves, transcendence, Diophantine approximation, and recurrence sequences. He had dozens of co-authors, and he especially liked to write with his Macquarie colleague John Loxton and with Bordeaux mathematician Michel Mendès France. Alf wrote a book, Notes on Fermat's Last Theorem, which was published by the CMS in 1996, and was co-author (with Everest, Shparlinski, and Ward) of another, Recurrence Sequences, published by the AMS in 2003. We had vague plans to write a book together on continued fractions, but we never did. We did, however, write four papers together.
Among number theorists, Alf was an "original fini", as the French say. His talks were often accompanied by transparencies lettered in Alf's distinctive hand, full of quotations and puns. (A book that he co-edited in honor of the computational number theorist Hugh Williams was called High Primes and Misdemeanours.) Some of his papers contained limericks -- and not always in good taste! One of his most-cited papers was just called "Folds!". He loved the mathematical typesetting system TeX passionately, and developed a large set of his own arcane macros, which sometimes made collaboration difficult.
Alf was born during World War II to Jewish parents in Holland who gave him up to another family to protect him from the Nazis. But they all survived the Holocaust, and Alf was reunited with his parents after the war. They moved to Australia around 1950, and Alf got a Ph. D. in mathematics from the University of New South Wales in 1968. He taught at Macquarie University for many years, and, after retirement, ran his own Centre for Number Theory Research out of his home in suburban Sydney. Alf spoke English with a distinctive mix of both Dutch and Australian accents. More information about his life can be found here.
Alf and his wife Joy welcomed me during my only visit to Australia back in 1991. I spent a month living on the campus of Macquarie University and working with Alf on continued fractions. One of the papers we wrote concerned the continued fraction expansion of numbers such as 2-1 + 2-2 + 2-3 + 2-5 + 2-8 + ..., where the exponents are the Fibonacci numbers. Working with Alf was a lot of fun. He would gesture languidly in the air with a cigarette-laden hand, moving quickly from topic to topic: Australian politics, other mathematicians he had known, an idea for our latest proof. Alf knew a lot about many different subjects and had a quip or story ready for almost anything. He had phenomenal mathematical intuition and nearly always found the right path to a solution, but he wasn't always rigorous when he wrote up his results. One of his celebrated papers, about the transcendence of automatic numbers, contained a serious flaw which he was never able to repair.
Alf travelled a lot. We worked together at Dartmouth College, and later, when I moved to the University of Waterloo, he came for one of the CMS meetings. During one of his visits, he told me about his clogged arteries. He had been walking along when suddenly he found himself unable to speak. He sat down on a bench and was taken to the hospital, where it was found that an important artery was nearly completely blocked. After surgery he made a full recovery and continued to do mathematics. More recently, he had surgery for cancer of the lip (where a cigarette had always rested in the past) and eventually developed the brain cancer that killed him. At one of the CMS meetings dozens of mathematicians signed a card for him.
Alf was active in the number theory community and served as president of the Australian Mathematical Society from 1996 to 1998 and won several awards, including the George Szekeres medal. I will miss him.
[Photo credit: Renate Schmid]