Carl Gustav Axel Harnack
Carl Gustav Axel Harnack (May 7, 1851, Tartu – April 3, 1888, Dresden) was a Baltic German mathematician who contributed to potential theory. Harnack's inequality applied to harmonic functions. He also worked on the real algebraic geometry of plane curves, proving Harnack's curve theorem for real plane algebraic curves.
He was the son of the theologian Theodosius Harnack and the twin brother of theologian Adolf von Harnack (who long outlived him) - all of them from Tartu in what is today Estonia.
* Harnack's inequality
* Harnack's principle
Books by A. Harnack
* Die Grundlagen der Theorie des logarithmischen Potentiales und der eindeutigen Potentialfunktion in der Ebene (Teubner, 1887)
* An introduction to the study of the elements of the differential and integral calculus Cathcart, George Lambert, tr. (Williams and Norgate, 1891)
* Axel Harnack at the mathematical genealogy project
* Moritz Kassman, Harnack Inequalities: An Introduction Boundary Value Problems (2007), 81415. (An article on Harnack's inequality that contains a biography of Axel Harnack in the introduction).
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