In the summer of that year Lars was in Europe and went to visit H. Falkenhagen, the Austrian electrochemist. Falkenhagen was unwell at the time and asked his sister, Gretl (Margarethe Arledter), to entertain Lars. Gretl saw him coming up the stairs—a very handsome young man who, her brother had told her, was "well ahead of his times." They went out to dinner, but Lars was his usual reticent self. After dinner he fell asleep on the patio, then woke up and said: "Are you romantically attached?" They became engaged eight days later—Margarethe at twenty-one and Lars at twenty-nine—and got married on September 7, 1933.

In 1933 Onsager was appointed Sterling and Gibbs Fellow at Yale University, where he was to remain for the greater part of his life. Having awarded him a postdoctoral fellowship, the Chemistry Department was embarrassed to discover that he had no Ph.D.

The Reciprocal Relations had been published two years before, but an outline of his results, submitted to his alma mater in Trondheim, had been judged unacceptable for a doctorate as it stood. This setback naturally upset him, and he felt uncomfortable at being addressed as "Dr. Onsager" when he had no such entitlement. His colleagues suggested he try for a Yale Ph.D. Lars had already taken the relevant course work and orals, and as for the thesis, any of his published work would do.

Lars felt, however, that he should write something new. It was soon ready—a thesis entitled Solutions of the Mathieu Equation of Period 4π and Certain Related Functions (1935,1). The chemists were quite out of their depth, and the physicists, too. Eventually the thesis was sent over to the Mathematics Department, where Professor Einar Hille, himself an expert in the subject matter of Whittaker and Watson's