A German mathematician (1815-1897), who was responsible for the rigorous definitions now in use in analysis for such things as limit, continuous, differentiable, and convergent. One of his main fields was the use of power series for complex functions.

Karl Wilhelm Theodor Weierstrass was born in Ostenfelde on 31 October 1815 and studied law in Bonn, but then became a schoolteacher. He did research in mathematics on the side, and came to prominence in 1854 with a paper on Abelian functions, after which he achieved a professorship at Berlin.

One of Weierstrass's pupils and collaborators was the renowned Russian mathematician Sonya Kovalevskaya, who achieved brilliant results with him despite her inability, as a woman, to be accepted for the normal path of study and teaching. He intensely disliked the idea of female students, and gave her a big bunch of dead hard sums to get rid of her, but realized her talent when she returned to him with answers.

He died in Berlin on 19 February 1897. He was a talented fencer, and disliked music. He disputed with Kronecker, a constructivist, because his arithmetic formulation of analysis sometimes used non-constructive proofs: once friends, this issue caused a serious rift between them.