The Bessel-Hagen reference in Kerékjártó's book has become a kind of urban legend among mathematicians and mathematics students. For instance some people think it is inspired by anti-Semitism. This is false: Bessel-Hagen was not Jewish and retained his university position in Bonn throughout the Nazi period. Another legend is that Bessel-Hagen was a strange character whose original name was Hagen, and that he added "Bessel-" to his name in order to preserve the name of the famous German astronomer/mathematician Friedrich Wilhelm Bessel. This is a garbled version of the truth: it was his grandfather, Bessel's son-in-law, who changed his own name from "Hagen" to "Bessel-Hagen" to preserve Bessel's name, since the latter had died without leaving any male heirs.
Briefly, Erich Bessel-Hagen was born near Berlin on September 12, 1898. He attended the University of Berlin, where he obtained his doctorate under the direction of Constantin Carathéodory in 1920. He then obtained a position as Privatdozent at the University of Göttingen. There he worked closely with Felix Klein, helping Klein to finish the editing of his collected works, just prior to his death in 1925. At the same Bessel-Hagen was finishing his habilitation work on elliptic modular functions. In 1927 he moved to the University of Halle, where he worked closely with Helmut Hasse. In 1929 he moved to Bonn where he worked closely with Otto Toeplitz, mainly on research in the history of mathematics. He remained in Bonn until his death on March 29, 1946.
Bessel-Hagen had wide-ranging mathematical interests, including calculus of variations, function theory, number theory, mathematical physics and history of mathematics. He is cited by many authors, including such eminent mathematicians as Emmy Noether, for helpful remarks and other informal contributions. Bessel-Hagen was also a gifted philologist with an impressive command of Greek, Latin and Arabic, which he put to good use in his researches on the history of mathematics. He also worked energetically to collect and preserve the correspondence and unpublished works of contemporary mathematicians, such as Klein, Hausdorff, and Toeplitz. Bessel-Hagen was generally regarded by his contemporaries as a very talented mathematician, who unfortunately did not realize his full potential due to his personality traits.
The problem was Bessel-Hagen's pathological shyness and lack of self-confidence. Consequently he was extremely reluctant to publish his own work, at great detriment to his career. Bessel-Hagen was also physically frail, awkward and slow talking. These qualities made him the natural target of various practical jokes by students and younger colleagues. Keréjártó was a visitor at Göttingen in 1920-21, while working on his book on topology, during the time Bessel-Hagen was Privatdozent there.
The above picture was taken during a mathematics conference in 1920. In the back row standing from left to right: Issai Schur, George Pólya , Erich Bessel-Hagen. In the middle row seated left to right: Béla Keréjártó, L. E. J. Brouwer , Otto Szász , Edmund Landau . Seated on the ground in front: Hans Hamburger.