Karl Theodor Wilhelm Weierstrass

Karl Weierstrass was born into a Roman Catholic family in Ostenfelde, a village near Ennigerloh, in the Province of Westphalia. He was the son of Wilhelm Weierstrass, a government official, and Theodora Vonderforst both of whom were Catholic Rhinelanders. His interest in mathematics began while he was a gymnasium student at the Theodorianum in Paderborn. He was sent to the University of Bonn upon graduation to prepare for a government position. Because his studies were to be in the fields of law, economics, and finance, he was immediately in conflict with his hopes to study mathematics. He resolved the conflict by paying little heed to his planned course of study but continuing private study in mathematics. The outcome was that he left the university without a degree. He then studied mathematics at the Münster Academy (which was even then famous for mathematics) and his father was able to obtain a place for him in a teacher training school in Münster. Later he was certified as a teacher in that city. During this period of study, Weierstrass attended the lectures of Christoph Gudermann and became interested in elliptic functions. In 1843 he taught in Deutsch Krone in West Prussia and since 1848 he taught at the Lyceum Hosianum in Braunsberg. Besides mathematics he also taught physics, botany, and gymnastics. Weierstrass may have had an illegitimate child named Franz with the widow of his friend Carl Wilhelm Borchardt. In 1854 he received an honorary doctorate from the University of Königsberg and became professor of mathematics at the Gewerbeinstitut (Institute of Trade), which later became the Technical University of Berlin. In 1864 he became professor at the Friedrich-Wilhelms-Universität Berlin (now Humboldt University), where he stayed until his retirement in 1892. Weierstrass was one of the founders of the modern theory of functions and made significant contributions to complex analysis, calculus, number theory, differential equations, approximation theory, and variational calculus. He formalized the definition of continuity of a function and introduced the concept of uniform convergence. He proved the intermediate value theorem and the Bolzano–Weierstrass theorem using rigorous epsilon-delta methods. He also developed the theory of analytic functions using power series and discovered examples of continuous but nowhere differentiable functions. He mentored many famous mathematicians such as Georg Cantor, Sofia Kovalevskaya, Leo Königsberger, Ernst Kötter, Mathias Lerch, Hans von Mangoldt, Arthur Schoenflies, Hermann Schwarz, and Ludwig Stickelberger. Weierstrass suffered from a nervous breakdown in 1871 and from pneumonia in his later years. He died on 19 February 1897 in Berlin at the age of 81. He was buried at St. Matthäus Kirchhof Cemetery in Berlin-Schöneberg. His grave is marked by a simple stone with his name and dates.