On one of my summer visits to Harwell, Dr Tony Skyrme encouraged me to work with Dr Euan Squires on the Brueckner t-matrix, about which I knew nothing. Over a 2-week period, I asked Euan about 50 questions, most of them trivial. He was able to answer 48 of these, but the other two led us to extend the theory (ref 7): this involved isolating a Delta-function singularity in certain singular linear integral equations, and showing how this singularity showed up in numerical computations. On this visit, I also got to know John Bell; our friendship continued until his death. 
 
In 1961, Dr Walter Marshall, then Head of the Theoretical Group at the Rutherford laboratory, invited me to visit. Dr John Gammel, visiting from Los Alamos, wanted to use my Feynman graph computational methods to evaluate fourth-order matrix elements, and to sum the mesonic strong-coupling perturbation series using Pade approximants. When we met, John Gammel launched into a 7-minute introduction to Pade approximants (of which I knew nothing), finishing, in his terse Texan way, with 'Well, there it is'. In 7 minutes, he had convinced me of the potential of the Pade method, and it was the basis of much of my work for the next sixteen years. What is surprising is that John and I have never collaborated on a paper. 
 
At coffee one morning at the Rutherford, I was struck by a revelation. In quick succession, two smartly dressed physicists of around my age arrived, each with a small suitcase. They were warmly greeted by the company, and it turned out that each had been visiting Universities and Institutes in far-flung places such as Bombay, Rome, New York and California. I asked myself why on earth I had not travelled around, and immediately collected from the secretariat a form of application to visit CERN in Geneva for a year. My application was successful, and Cardiff UC gave me leave of absence for the academic year 1962-63. 
 
In the summer of 1962, I was offered a chair of Natural Philosophy (meaning applied maths/theoretical physics) at Trinity College, Dublin. Nobel Prizewinner Professor Cecil Powell of Bristol gave me very strong advice to put CERN before Trinity College, saying 'Going to CERN will change your life, and that of your family', a prophetic utterence. However, TCD offered to let me start with a year on leave of absence. I was very dubious about moving to Dublin, but after weeks of thought, my wife Monty and I decided that I should leave Cardiff, and go to TCD in 1963. It was a finely-balanced decision, and if we had known all the facts, I would probably not have accepted the Chair. But we cannot say that the decision was wrong, because we can never know what would have happened in the 'parallel world' in which I declined the offer.