In 1968 he was invited to attend a conference on Dynamical Systems at the University of Warwick. These major conferences at Warwick had many international visitors throughout an academic year with periods of higher activity. They have been important in the development of many mathematical topics.
Peter had been involved in politics in Czechoslovakia, supporting the political movement which hoped to humanise Communist rule by introducing basic civil freedoms, an independent judiciary, and other democratic institutions. During Peter's visit to Warwick the Soviets invaded Czechoslovakia on the night of August 20-21, 1968, installing a Soviet controlled security service. Peter feared that he would be in danger if he returned, and since the reform programme had stopped, he preferred the freedom in Britain.
Stefan remained at the University of Warwick where he studied for a Ph.D. which was awarded in 1973. His thesis was on Accessibility and singular foliations and is important in control theory and in the mathematical theory of entropy.
However Stefan did not remain at Warwick while working for his doctorate. He spent 1969/70 at Manchester returning for a year to Warwick before being appointed to a lectureship at the University College of North Wales at Bangor, a post he held until his death at the age of 37. He spent one year in Paris during his tenure of the Bangor post, spending 1976/77 at the Institut des Hautes Études Scientifique.
Stefan had a love of freedom and he translated this into a love of climbing after his return from Paris. He was killed in a climbing accident in Snowdonia. He was climbing on his own at the time.
In  his attitude towards mathematics is summed up as follows:-
Peter had a strong sense of what was important in mathematics ... Given a first-rate mathematical idea, he made it part of himself. That often required an exhaustive search for the right perspective in mathematical development, in exposition, technical accuracy, and historical viewpoint. His fine taste and judgement shine throughout his work.
Article by: J J O'Connor and E F Robertson