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Jakob Steiner (18 March 1796 – 1 April 1863) was a Switzerland mathematician who worked primarily in geometry.
After Steiner's publication (1832) of his Systematische Entwickelungen he received, through Carl Gustav Jacob Jacobi, who was then professor at Königsberg University, and earned an honorary degree there; and through the influence of Jacobi and of the brothers Alexander and Wilhelm von Humboldt a new chair of geometry was founded for him at Berlin (1834). This he occupied until his death in Bern on 1 April 1863.
He was described by Thomas Hirst as follows:
In his Systematische Entwickelung der Abhängigkeit geometrischer Gestalten von einander he laid the foundation of modern synthetic geometry. In projective geometry even parallel lines have a point in common: a point at infinity. Thus two points determine a line and two lines determine a point. The symmetry of point and line is expressed as projective duality. Starting with perspectivity, the transformations of projective geometry are formed by composition, producing projectivities. Steiner identified sets preserved by projectivities such as a projective range and pencils. He is particularly remembered for his approach to a conic section by way of projectivity called the Steiner conic.
In a second little volume, Die geometrischen Constructionen ausgeführt mittels der geraden Linie und eines festen Kreises (1833), republished in 1895 by Ottingen, he shows, what had been already suggested by J. V. Poncelet, how all problems of the second order can be solved by aid of the straight edge alone without the use of compasses, as soon as one circle is given on the drawing-paper. He also wrote "Vorlesungen über synthetische Geometrie", published posthumously at Leipzig by C. F. Geiser and H. Schroeter in 1867; a third edition by Rudolf Sturm was published in 1887–1898.
Other geometric results by Steiner include development of a formula for the partitioning of space by planes (the maximal number of parts created by n planes), several theorems about the famous Steiner's chain of tangential circles, and a proof of the isoperimetric theorem (later a flaw was found in the proof, but was corrected by Weierstrass).
The rest of Steiner's writings are found in numerous papers mostly published in Crelle's Journal, the first volume of which contains his first four papers. The most important are those relating to and surfaces, especially the short paper Allgemeine Eigenschaften algebraischer Curven. This contains only results, and there is no indication of the method by which they were obtained, so that, according to Otto Hesse, they are, like Fermat's theorems, riddles to the present and future generations. Eminent analysts succeeded in proving some of the theorems, but it was reserved to Luigi Cremona to prove them all, and that by a uniform synthetic method, in his book on algebraic curves.
Other important investigations relate to maximum and minimum. Starting from simple elementary propositions, Steiner advances to the solution of problems which analytically require the calculus of variations, but which at the time altogether surpassed the powers of that calculus. Connected with this is the paper Vom Krümmungsschwerpuncte ebener Curven, which contains numerous properties of Pedal curve and roulettes, especially of their areas.
Steiner also made a small but important contribution to combinatorics. In 1853, Steiner published a two-page article in Crelle's Journal on what nowadays is called , a basic kind of block design.
His oldest papers and manuscripts (1823–1826) were published by his admirer Fritz Bützberger on the request of the Bernese Society for Natural Scientists.
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