In 1894 he entered the University of Göttingen intending to follow in his father's footsteps and to study medicine. However, after one semester studying medicine he switched to study mathematics and science. He spent the summer semester at Munich where he attended lectures by Lindemann and Pringsheim, and then returned to Göttingen. He had some outstanding teachers at Göttingen, where his lecturers included Schönflies, Hilbert and Klein, and it was in this outstanding mathematical environment that Blumenthal undertook research. He was much influenced by Sommerfeld, at this time Klein's assistant, and although formally Blumenthal is considered as Hilbert's first research student, nevertheless his work was also guided by Sommerfeld. Reid, in , describes Blumenthal at this time as:-
... a gentle, fun-loving, sociable young man who spoke and read a number of languages and was interested in literature, history and theology as well as mathematics and physics.This gentle, fun-loving, sociable young man could not have dreamt at this time of the hardships that life had in store for him. After submitting his doctoral dissertation, Blumenthal took his oral examination on 25 May 1898 and passed with distinction. His thesis was on Stieltjes continued fraction expansions, a topic which he continued to develop in his later work. Remaining at Göttingen, he qualified to teach in secondary schools after taking the necessary examinations in 1899. After this he went to Paris where he spent the winter of 1899-1900 studying under Borel and Jordan. While he was in Paris he became interested in the theory of entire functions and he wrote a number of papers in the following years which made major contributions to this theory. Returning to Göttingen, he submitted his habilitation thesis Über Modulfunktionen von mehreren Veränderlichen in 1901. Van der Geer writes in  that in this work:-
... Blumenthal did the first pioneering work in a programme outlined by Hilbert with the aim of creating a theory of modular functions of several variables that should be just as important in number theory and geometry as the theory of modular functions of one variable was at the beginning of the [20th] century.Having submitted his habilitation thesis Blumenthal could now lecture at Göttingen as a Privatdozent, which he did from the autumn of 1901 until the summer of 1905. He also substituted for the professor at Marburg.
Sommerfeld has been appointed as professor of mechanics at the Technische Hochschule in Aachen in 1900. He knew Blumenthal's high capabilities and when a position in Aachen became available he recommended Blumenthal. Indeed Blumenthal was appointed to a Chair of Mathematics at the Rheinisch-Westfälische Technische Hochschule Aachen in October 1905. He went on to hold important positions at the University, being elected to the senate in 1914-15 and again in 1921, being dean in 1927-28, and senate representative for the Faculty of Science in 1927-28. Three years after he took up the position in Aachen, Blumenthal married Mali Ebstein, the daughter of pathologist Wilhelm Ebstein and his wife Elfriede. Otto and Mali had two children, a daughter Margrete born in 1911 and a son Ernst born in 1914.
During World War I, Blumenthal took charge of military weather stations and worked at an aircraft firm in 1918. After the war ended Blumenthal made efforts to ensure that German mathematicians were integrated back into the international mathematical scene. He also became a member of both the German League for Human Rights and the Society of Friends of the New Russia, and he actively played a role in promoting peace. This would later be considered a crime by the National Socialists.
One of the major roles that Blumenthal played in mathematics, outside his research contributions which we shall look at briefly throughout in this article, was his role as executive editor of the journal Mathematischen Annalen. He took on this position in 1905 and devoted much effort to it over several decades. In 1926 Blumenthal was 50 years old and his mathematical friends organised a birthday present to thank him for his contributions to Mathematischen Annalen. Blumenthal was given the works of Gauss, nicely bound:-
... for the great pains he had taken during twenty years on the board and for the qualities he brought to that journal.In fact Blumenthal had also taken on a second major editorial role two years before this when, in 1924, he became editor of the Jahresberichte der Deutschen Mathematiker-Vereinigung. This meant that he became one of the three executive members of the German Mathematical Society.
Blumenthal's life changed abruptly after the National Socialists came to power on 30 January 1933. On 1 April there was the so-called "boycott day" when Jewish shops were boycotted and Jewish lecturers were not allowed to enter their university. On 7 April the Civil Service Law provided the means of removing Jewish teachers from the universities, and of course also to remove those of Jewish descent from other roles. All civil servants who were not of Aryan descent (having one grandparent of the Jewish religion made someone non-Aryan) were to be retired. On 27 April Blumenthal was arrested and detained. He had been denounced as a communist by the Aachen Student Association, certainly a false accusation, and after two weeks he was released but he was suspended from his teaching duties at the university. The official reasons were not racial, but rather cited his involvement with the German League for Human Rights and the Society of Friends of the New Russia.
Following Blumenthal's suspension, Bieberbach apears to have put pressure on him to resign as editor of the Jahresberichte der Deutschen Mathematiker-Vereinigung which he did. He must also have been influenced by the internal arguments in the German Mathematical Society by Doetsch, Hamel, Blaschke and Bieberbach. On 22 September his dismissal from his university post was confirmed. Blumenthal and his wife continued to live in Aachen but their son Ernst was sent to England to study in Manchester. Margrete, their daughter, was able to complete her doctorate in Cologne and in 1936 she too went to England. Blumenthal continued with his role of executive editor of Mathematischen Annalen until 1938 when he finally was prevented from continuing any work. A law was also passed which meant that scientific societies could only have German members (as defined by the civil law) and, after many years of outstanding service, Blumenthal's membership of the German Mathematical Society came to an end.
In July 1939 Blumenthal was given permission to privately tutor students at the Technical University of Delft. This was valuable in allowing him to obtain the necessary immigration documents for the Netherlands, but he still had many difficulties to overcome which were placed in the way of Jews leaving Germany. The Blumenthals arrived in the Netherlands on 13 July and were given refuge in a house near Utrecht. From Utrecht they were able to make a trip to England to visit their children and they returned to the Netherlands only days before the start of World War II. On 19 October 1939 they moved to Delft having found a flat there. However, Blumenthal only found one Delft student to tutor and he had to live on charity, at first provided by the Protestant Hulpcomite. After the German invasion of the Netherlands, they were forced to leave Delft in September 1940 when ethnic cleansing made it a non-Ayran free zone. They went to Utrecht but were ordered to move house repeatedly. On 23 November 1940, all Jewish professors of the Utrecht University were dismissed, among them the Dutch mathematician Julius Wolff. About two weeks later Wolff and Blumenthal established a weekly mathematical colloquium of their own. It was an ordinary mathematical colloquium, the members gave, in turn, carefully prepared talks. The remarkable thing was that there were just two members, Wolff and Blumenthal, and the colloquium took place in their private rooms and ran for nearly two years until October 1942.
After months of increasing humiliation and harassment imposed on Jews, such as having to wear the star of David, not being allowed to use buses, and not being allowed to visit non-Jewish friends, it was announced in April 1943 that Utrecht would be ethnically cleansed of Jews. The Blumenthals were sent to the concentration camp at Westerbork where Mali Blumenthal died following her humiliating treatment. Blumenthal wrote to a friend:-
I thank God that he gave me a bearable life, but also that he relieved my wife so early. For her that was intolerable which I accepted quietly. Looking back at the past year, it was painful but quiet.On 20 January 1944 Blumenthal was sent, at his own request, to the "old people's ghetto" Theresienstadt since he had heard that his sister had been sent there in July 1942. When he arrived at Theresienstadt he found that, although his sister had been there, she had died six months earlier. Blumenthal himself died at Theresienstadt after suffering from pneumonia, dysentery and tuberculosis.
We have given a brief indication above of some of the mathematical topics Blumenthal studied. His basic interests were in complex function theory, but he also did much work in applying his theory to a wide variety of different applied mathematics problems. In particular he considered problems of stress in airplane wings, vibration of membranes, and tension in beams.
Article by: J J O'Connor and E F Robertson