Workshop on the occasion of Prof. Heinz W. Engl's 60th Birthday
Charles Groetsch
“Remarks on the prehistory of regularization theory”
In and around the year 1907 Charles Émile Picard was the pivot point for a remarkable array of fundamental results in what would become the theory of Hilbert space operators. We review some of his interactions in 1907 and present a speculative reconstruction of the path to his seminal paper of 1910. Some developments during the following half century that led to the invention of regularization theory are then discussed.