Karl Theodor Wilhelm Weierstrass (German: Weierstraß; 31 October 1815 – 19 February 1897) was a German mathematician often cited as the "father of modern analysis". Despite leaving university without a degree, he studied mathematics and trained as a teacher, eventually teaching mathematics, physics, botany and gymnastics.
Weierstrass formalized the definition of the continuity of a function, and used it and the concept of uniform convergence to prove the Bolzano–Weierstrass theorem and Heine–Borel theorem.
Contents

Biography 1

Mathematical contributions 2

Soundness of calculus 2.1

Calculus of variations 2.2

Other analytical theorems 2.3

Selected works 3

Students 4

Honours and awards 5

See also 6

References 7

External links 8
Biography
Weierstrass was born in Ostenfelde, part of Ennigerloh, Province of Westphalia.^{[1]}
Weierstrass was the son of Wilhelm Weierstrass, a government official, and Theodora Vonderforst. His interest in mathematics began while he was a Gymnasium student at 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 continued private study in mathematics. The outcome was to leave the university without a degree. After that he studied mathematics at the University of Münster (which was even at this time very 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 DeutschKrone in Westprussia and since 1848 he taught at the Lyceum Hosianum in Braunsberg. Besides mathematics he also taught physics, botanics and gymnastics.^{[1]}
Weierstrass may have had an illegitimate child named Franz with the widow of his friend Carl Wilhelm Borchardt.^{[2]}
After 1850 Weierstrass suffered from a long period of illness, but was able to publish papers that brought him fame and distinction. In 1856 he took a chair at the Gewerbeinstitut, which later became the Technical University of Berlin. In 1864 he became professor at the FriedrichWilhelmsUniversität Berlin, which later became the Humboldt Universität zu Berlin. He was immobile for the last three years of his life, and died in Berlin from pneumonia.
Mathematical contributions
Soundness of calculus
Weierstrass was interested in the soundness of calculus, and at the time, there were somewhat ambiguous definitions regarding the foundations of calculus, and hence important theorems could not be proven with sufficient rigour. While Bolzano had developed a reasonably rigorous definition of a limit as early as 1817 (and possibly even earlier) his work remained unknown to most of the mathematical community until years later, and many had only vague definitions of limits and continuity of functions.
Deltaepsilon proofs are first found in the works of Cauchy in the 1820s.^{[3]}^{[4]} Cauchy did not clearly distinguish between continuity and uniform continuity on an interval. Notably, in his 1821 Cours d'analyse, Cauchy argued that the (pointwise) limit of (pointwise) continuous functions was itself (pointwise) continuous, a statement interpreted as being incorrect by many scholars. The correct statement is rather that the uniform limit of continuous functions is continuous (also, the uniform limit of uniformly continuous functions is uniformly continuous). This required the concept of uniform convergence, which was first observed by Weierstrass's advisor, Christoph Gudermann, in an 1838 paper, where Gudermann noted the phenomenon but did not define it or elaborate on it. Weierstrass saw the importance of the concept, and both formalized it and applied it widely throughout the foundations of calculus.
The formal definition of continuity of a function, as formulated by Weierstrass, is as follows:
\displaystyle f(x) is continuous at \displaystyle x = x_0 if \displaystyle \forall \ \varepsilon > 0\ \exists\ \delta > 0 such that for every x in the domain of f, \displaystyle \ xx_0 < \delta \Rightarrow f(x)  f(x_0) < \varepsilon.
Using this definition and the concept of uniform convergence, Weierstrass was able to write proofs of several theorems such as the intermediate value theorem, the Bolzano–Weierstrass theorem, and Heine–Borel theorem.
Calculus of variations
Weierstrass also made significant advancements in the field of calculus of variations.
Other analytical theorems


See also List of things named after Karl Weierstrass.
Selected works

Zur Theorie der Abelschen Funktionen (1854)

Theorie der Abelschen Funktionen (1856)

Abhandlungen1// Math. Werke. Bd. 1. Berlin, 1894

Abhandlungen2// Math. Werke. Bd. 2. Berlin, 1895

Abhandlungen3// Math. Werke. Bd. 3. Berlin, 1903

Vorl. ueber die Theorie der Abelschen Transcendenten// Math. Werke. Bd. 4. Berlin, 1902

Vorl. ueber Variationsrechnung// Math. Werke. Bd. 7. Leipzig, 1927
Students
Honours and awards
The lunar crater Weierstrass is named after him.
See also
References

^ ^{a} ^{b} O'Connor, J. J.; Robertson, E. F. (October 1998). "Karl Theodor Wilhelm Weierstrass". School of Mathematics and Statistics, University of St Andrews, Scotland. Retrieved 7 September 2014.

^ Biermann, KurtR.; Schubring, Gert (1996). "Einige Nachträge zur Biographie von Karl Weierstraß. (German) [Some postscripts to the biography of Karl Weierstrass]". History of mathematics. San Diego, CA: Academic Press. pp. 65–91.

^ Grabiner, Judith V. (March 1983), "Who Gave You the Epsilon? Cauchy and the Origins of Rigorous Calculus" (PDF), The American Mathematical Monthly 90 (3): 185–194,

^
External links

.

Karl Weierstrass at the Mathematics Genealogy Project

Digitalized versions of Weierstrass's original publications are freely available online from the library of the Berlin Brandenburgische Akademie der Wissenschaften.

Works by Karl Weierstrass at Project Gutenberg

Works by or about Karl Weierstrass at Internet Archive
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