It was the applied mathematician L Lichtenstein who became Schubert's thesis advisor in 1933 and under his guidance Schubert wrote a thesis Über einige Lichtensteinsche Hilfssätze der Potentialtheorie und ihre Anwendung auf die Hydrodynamik on potential theory and its applications to hydrodynamics. The thesis was submitted to the University of Leipzig in 1935 and he was awarded his doctorate on 14 January 1936. On 1 April of that year Schubert was appointed as an assistant to H Schmidt in Köthen, a position which he held until October. During this time, on 23 April, he took the state examinations to qualify to teach in schools. The essay which he submitted for this examination was Die Mitwirkung mathematischer Begriffe im Aufbau von Leibniz' Metaphysik (Darstellung und Kritik) .
The years during which Schubert was undertaking research were the years during which there were huge changes in Germany. Hitler came to power in 1933 and within a year had proclaimed himself Chancellor and Führer of the German Reich. Germany had no air force since it was forbidden under the 1919 treaty of Versailles signed after World War I. Hitler now ordered the creation of a German air force and began to rapidly expand the army and navy. In January 1935 he regained the Saar region by holding a plebiscite, and in March 1936 German troops entered the Rhineland. In August 1936 two year conscription was declared in Germany. Schubert, with his expertise in hydrodynamics, was exactly the right person to work on aviation research and from 1 November 1936 until 1 April 1945 he undertook research at the aviation laboratory in Berlin-Adlershof.
Now working in Berlin, Schubert took part in the mathematical activities that the city offered. He joined the German Mathematical Society and the Berlin Mathematical Society, both in 1940. In 1943 he submitted his habititation thesis Zur Berechnung der Abwindkorrektur in der Strahlachse eines Windkanals von kreisförmigem Querschnitt bei Berücksichtigung einer offenen Messtrecke to the Technische Hochschule in Darmstadt. Schubert had published eight papers between his doctoral thesis and his habititation thesis. These papers included Über einige Eigenschaften elliptischer und nichtelliptischer Tragflügel (1938) and Zur Ermittlung der Auftriebsverteilung nichtelliptischer Tragflügel mit der Methode der sukzessiven Approximationen (1938) which were specifically related to his aviation research. Other papers such as Ein potentialtheoretischer Hilfssatz (1940), published in Mathematische Zeitschrift, related to the work he had done on potential theory for his doctorate.
After World War II ended, Schubert taught mathematics and physics at the Oberrealschule in Crimmitschau from 1 October 1945 to the 31 January 1947. However during the summer vacation in 1946 he was a guest lecturer at the University of Leipzig. He was appointed as an extraordinary professor of mathematics at the University of Rostock and was due to take up his duties in the autumn of 1946 but problems with heating at the university meant that it was closed until the spring of 1947. Schubert, therefore, continued in his position as a school teacher until the end of January, taking up his duties at Rostock on 1 March 1947, and lecturing on Why applied mathematics? during the summer semester. On 21 August he married Käte Falkenberg who was a teacher from Crimmitschau.
Over the next two years Schubert turned down offers of posts at the University of Jena and at the Bergakademie in Freiberg. He was appointed a full professor at the University of Rostock on 1 April 1950 but on 7 September 1951 Harry Schmidt, who held the chair of mathematics at the Martin-Luther University of Halle-Wittenberg, died and Schubert accepted the offer of the post, taking up his duties on 1 October 1952. He remained at Halle for the rest of his career, being named Professor of Applied Mathematics in 1960 and Professor of Analysis in 1969. Because of poor health, he retired early on 1 January 1970.
Let us look briefly at some of the papers which Schubert published. Über eine lineare Integrodifferentialgleichung mit Zusatzkern (1950) looked at certain aerodynamical problems which lead to integrodifferential equations. In Über ein gemischtes räumliches Randwertproblem der Potentialtheorie I (1951) he considered the uniform flow of an incompressible ideal fluid past a small obstacle in an infinite cylindrical channel. Under certain conditions, he obtains solutions in terms of Bessel and Hankel functions and computes the resulting formulas for downwash. He continued this investigation in a second paper. In Über die Potentiale der auf dem Mantel eines Kreiszylinders ausgebreiteten einfachen und doppelten Belegung (1952) he derives a Fourier integral representation containing Bessel functions for the axially symmetric potential induced by a simple and double layer on the surface of a circular cylinder. He wrote two papers with Erich Schincke, the first being Zum Konturproblem der Hodographenmethode im Unterschall (1959) in which they consider the problem of subsonic flow without circulation past a body of prescribed shape and symmetric about two perpendicular axes; the second paper being Die Berechnung einer zirkulationslosen Unterschallströmung um den Kreiszylinder mit der Hodographenmethode (1959) in which they use a modification of the hodograph method to calculate an approximation to the ideal gas flow past a circular cylinder. In 1970, the year he retired, Schubert published a survey of the literature on the Poincaré boundary value problem.
At Halle Schubert taught a variety of different courses such as differential and integral calculus, partial differential equation, and integral equations. He was honoured in several ways. He was a member of the German Society for Applied Mathematics and Mechanics (1948), and elected to the German Academy of Scientists Leopoldina (1959). He was co-editor of the Zeitschrift für Angewandte Mathematik und Mechanik (ZAMM) from 1959 until his death.
Article by: J J O'Connor and E F Robertson