Parry was born in Coventry, where his father and three brothers were sheet-metal workers; only one of his three sisters went on to higher education, as a mature student at Coventry College of Education. But Bill was persuaded by a perceptive and enthusiastic maths teacher at Coventry junior technical school, specialising in metalwork and woodwork, to aim for university. To get appropriate tuition, he had to travel to Birmingham Technical College. Despite his subsequent achievements, and his diverse knowledge of science, literature, politics, poetry, cinema and the theatre, he felt this early educational deprivation keenly, and often expressed regret that his training had been narrow.
As a student at University College London, he impressed the mathematician Hyman Kestelman with his instinct for the subject. He missed a first-class degree through skipping lectures that did not interest him, and divided his time between selling the Daily Worker and losing money at poker. Following an MSc at Liverpool, he returned to London to study at Imperial College with Yael Dowker, obtaining his PhD in 1960. After a lectureship at Birmingham University (1960-65) and a senior lecturership at Sussex (1965-68), he was appointed to a readership at the recently created University of Warwick. Two years later, he gave a much acclaimed address at the four-yearly International Congress of Mathematicians, and was promoted to professor. In 1984 he was elected to the Royal Society.
A characteristic of Bill's work was tremendous care and attention to detail. Each paper was the product of numerous handwritten drafts. He spoke without notes, giving enthusiastic and exciting lectures, though sometimes retrieving from memory previously worked out technical details only with some difficulty. He introduced a number of important concepts which bear his name, such as the Parry-Daniels map, the Parry-Sullivan invariant and the Parry measure. The last of these was particularly influential, anticipating ideas from the modern theories of hyperbolic dynamical systems and thermodynamical formalism.
Bill played a founding role in the study of subshifts of finite type, and his work on nilflows was highly regarded. He was particularly interested in connections between ergodic theory and other areas of mathematics, especially number theory. His early work on the invariant density for beta transformations, first studied by the Hungarian mathematician Alfred Renyi, incorporated both of these themes, as did his more recent work on dynamical zeta functions.
His published works include more than 80 research articles and four books. He was a founder of the journal Ergodic Theory and Dynamical Systems, and worked with Cambridge University Press to ensure the publication of books in this area. Perhaps his most important scientific legacy was the establishment of a thriving school in this mathematically central area, previously almost unknown in the UK. His ability to attract world-class visitors helped to establish the international reputation of the Warwick mathematics department, and of his 20 doctoral students, the majority stayed in academia; a significant proportion now hold chairs at UK universities.
Bill disliked administrative work, claiming incompetence, and there was some anxiety prior to his stint as chair of the mathematics department (1984-86), a difficult and thankless task. For all that, he was commonly regarded as a successful administrator, overseeing some important changes in the department. He used to congratulate himself on his skill in negotiating a reduction from three years to two as departmental head.
Bill's father and brothers were active trade unionists, and many of his family were either fellow travellers or members of the Communist party, as was Bill himself. Following the repression of the Hungarian uprising in 1956, he broke with the CP and joined the Socialist Labour League (SLL), which he soon left, disillusioned with the leadership. But socialist values continued to imbue all his views; he was, for instance, an enthusiastic supporter of the collection of academic books for Palestinian universities, and went on supporting leftwing causes financially and morally.
While on the 1958 Aldermaston march, he met the woman who became his lifelong partner, Benita Teper, recently arrived from South Africa, where she had been involved in Trotskyite politics. Through Benita, who works in the field of postcolonial studies, Bill's intellectual life was further expanded and his circle of friends enlarged. He liked to work at home, in the Warwickshire village of Marton, in the company of his family and cats. In retirement, he pursued a long-standing interest in poetry, and had begun writing poems himself, some of which have been published.
Bill and Benita were famous for their dinner parties, and, in the 1970s, dancing parties full of wine, whisky, cigarettes, laughter and lively discussion, often about politics. Totally without pretence, Bill disliked pomposity and ceremony, and underestimated his own achievements, sometimes painfully so.
He was modest, unassuming and generous, ready to praise, enthuse about and encourage the work of others. He always had time to discuss ideas, mathematical or otherwise, often while smoking his favourite Gauloises or Gitanes sans filtre. Benita and their daughter Rachel survive him.
David Epstein and Mark Pollicott Friday October 13, 2006