Karl Theodor Wilhelm Weierstraß (31 October 1815 - 19 February 1897) was a German mathematician. Most of his work is about using logic as a thorough foundation of analysis. He mainly used power series for this. He also provided foundations for the theory of elliptic functions, differential geometry and the calculus of variations. The Bolzano-Weierstrass theorem is named in his honour: according to the theorem, finite series have certain properties: finite convergent series of complex numbers have at least one limit point, those of real numbers have at least two. He criticised earlier mathematicians for using assumptions too often, and being too vague in their definitions.