Obituaries Australia

Alf van der Poorten might well be described as a man on top of his subject, a mathematician who wrote some 180 papers on topics such as continued fractions, elliptic curves, transcendence, Diophantine approximation and recurrence sequences, as well as that most perplexing item, Fermat's Last Theorem, which had puzzled mathematicians from the time Frenchman Pierre de Fermat produced it in 1637.

Despite such weighty matters, van der Poorten was not above a joke, composing limericks that he put in his papers and titling a book on a fellow mathematician High Primes and Misdemeanours. A professor of pure mathematics at Macquarie University, van der Poorten was also a family man, computer enthusiast, crime-novel reader and St George rugby league fanatic. He retained his mathematical passion throughout his career, making headlines when he remarked on one occasion that mathematics was ''beautiful, elegant and fun, a language as worthwhile learning as any other''.

But when he received a letter from a man who claimed the just-discovered proof of Fermat's Last Theorem was an American conspiracy, he declared: ''I won't write back to him at all. He's mad!''

Alfred Jacobus van der Poorten was himself born in mad times, in a Jewish family in the Netherlands on May 16, 1942, when the Nazis had occupied his country. His parents, David and Marianne, delivered the infant Alf and his sister to foster families before going into hiding. His parents were later caught and sent to a concentration camp. For the next few years, living in Amersfoort, van der Poorten was known as ''Fritsje Teerink''. His parents were among the few who came back from the concentration camps and the family was reunited in 1945, then migrated to Sydney in 1951.

Living in Bellevue Hill, van der Poorten quickly adapted to Australian life, making pushbike deliveries for a local pharmacist. He developed a passion for St George and, after much negotiation, his parents agreed to let him paint his bedroom door red and white. He shone academically at Sydney Boys High and enrolled at the University of NSW (UNSW) to study science.

He was active in student politics, serving as president of the students' university council and the university union. He also served as the national president of the Jewish youth movement Betar, the Australasian Union of Jewish Students and the state Zionist Youth Council. Van der Poorten obtained a bachelor of science, a bachelor of arts (philosophy), an MBA and a PhD. In 1969, he was appointed a lecturer in mathematics at UNSW and a senior lecturer in 1972. That year, he married a former manager of the university's co-op bookshop, Joy FitzRoy.

Van der Poorten became an associate professor at UNSW in 1976. In 1979, he moved to Macquarie University, where he took up a full professorship. In 1980, he became the head of the university's school of mathematics, physics, computing and electronics and in the pursuit of academic interests made a trip to China, one of many such travels that were to take him and his family all over the world. Van der Poorten found a visit to Iran fascinating and also enjoyed regular visits to MSRI (pronounced ''misery'') in Berkeley, in the US, and delighted in telling colleagues he was ''currently in MSRI but would soon return to Joy''.

Van der Poorten spent a lot of time on his computer and read science fiction and mysteries.

He claimed never to have thrown a book away. He stepped down from the position as the head of department in 1987 but took it up again in 1991 for a further five years. In 1994-95, he chaired a working party on behalf of the National Committee for Mathematics to report on ''Mathematical Sciences Research and Advanced Mathematical Services in Australia''. He also was a member of the Macquarie University Academic Senate, the university's senior executive and an ex-officio member of the university council.

In 1996, his book Notes on Fermat's Last Theorem was published and he was elected president of The Australian Mathematical Society. In 1998, he joined the new Committee on Electronic Information and Communication of the International Mathematical Union. The Universite Bordeaux in France awarded him an honorary doctorate.

Also that year, van der Poorten was one of Australia's three delegates to the quadrennial Assembly of the International Mathematical Union. He was awarded the Australian Mathematical Society's George Szekeres Medal in recognition of his career research achievements and contribution.

In 2004, he was appointed a member of the Order of Australia. In 2006, van der Poorten made his third trip as a delegate of the Assembly of the International Mathematics Union.

He was ill this year with lung cancer and a brain tumour but lived long enough to see his beloved St George Illawarra Dragons once again win a premiership. Van der Poorten is survived by Joy, mother Marianne, sister Rose, children Kate and David and four grandchildren.