Karl Weierstrass. Portrait of Karl Wilhelm Theodor Weierstrass (1815-97), German mathematician. Sent by his father to read law at Bonn University, Weierstrass spent much of his time fencing and studying mathematics. He became a teacher in a small village school, and for 15 years developed a theory that power series could be used as a rigorous basis for mathematical analysis. The qua- lity of a paper he published in 1854 led to univ- ersity appointments. Weierstrass became a popular lecturer, gave the first rigorous definitions of the fundamental concepts of analysis, investi- gated power series, and made great contributions to function theory and Abelian functions.