Augustin Cauchy. Engraving of Baron Augustin Louis Cauchy (1789-1857), French mathematician. He did important work in differential equations, arguing the need to establish conditions such that they had solutions. He was the first to give the defin- itions of differentiability and continuity which are now standard. He also founded the theory of functions of a complex variable which played a leading role in mathematics during the rest of the 19th century. In algebra he developed the theory of determinants and the idea of permutation groups. His output was so prolific that he founded journals solely to publish the results of his research. Engraving by L. Ruet.