When first confronted with the prospect of having my collected papers published, I felt both awe and confusion, but I calmed down when I realized that the purpose was not to honor the author, but to be of service to the mathematical community. If young scholars of a future generation should desire to find out what some mathematicians of the twentieth century were up to, they would indeed have reason to be thankful if spared the need to seek this information from a multi­ tude of sources. As an introduction it seems polite and useful to begin with a brief outline of my life, especially as related to my professional activity. I was born the eighteenth of April 1907 in Helsingfors, Finland. My father was a professor of mechanical engineering at the Poly technical Institute. My mother died in childbirth when I was born. At the time of my early childhood Finland was under Russian sovereignty, but with a certain degree of autonomy, sometimes observed and sometimes disregarded by the czar who was, by today's standards, a relatively benevolent despot. Civil servants, including professors, were able to enjoy a fairly high standard of living, a condition that was to change radically during World War I and the Russian revolution that followed.