Lettere di Paolo Medolaghi a Friedrich Engel sulla teoria dei gruppi continui

In this work we present the annotated transcription of 13 letters from Paolo Medolaghi to Friedrich Engel preceded by an extensive introduction. These letters contribute to shedding light on the genesis of Medolaghi's ideas on the theory of continuous Lie groups and on the reasons why Medolaghi, after having obtained very significant results in the field, decided to leave pure mathematics and pursue a career as an official in Social Security institutions.