Tensions arose in the family because the Cornils side were Aryan and the Tietz side Jewish. On 1 January 1930, Amanda's brother Walter Cornils joined the Nazi Party increasing the tensions between the two sides. Added to these difficulties was the severe economic problems. The Great Depression hit most countries world-wide and the German economy went into decline in 1928 and did not begin to recover until the autumn of 1932. This depression caused major problems for Willi Tietz's wood business which hit desperate times. The problems suffered by Willi's brother Hermann were similar and he had to give up his pharmacy in Berlin in 1935, and then, in total despair, he took his own life. Willi Tietz would probably have taken the same way out of his problems had it not been for the strong support he received from his wife Amada.
In the Hamburg Realgymnasium, mathematics was the subject Horst loved but his plans changed when World War II broke out. After graduating from school with his Abitur at Easter 1939, Tietz was drafted into the Reichsarbeitsdienst (the Reich Labour Service). The Nazi government decreed that soldiers could take leave to continue their studies if they took courses in chemistry or medicine since these were considered war-related courses. He had no interest at all in medicine so, even though chemistry was not a subject he liked, he decided to enter university to study chemistry. At this time Hamburg University was closed, so he enrolled to study chemistry at the University of Berlin beginning his studies in January 1940. In Berlin he was able to live with Meta Tietz, the widow of his uncle Hermann who had committed suicide. Although he had a Jewish background, he was allowed to enrol since his father had fought for Germany in World War I. Chemistry courses did not prove to be a great success for him and when Hamburg University reopened, he was able to begin studying mathematics there in January 1940. He was taught by Erich Hecke, who held the chair of mathematics, and by his assistant Hans Zassenhaus. However, Tietz said he was always afraid of Wilhelm Blaschke, another of his professors at Hamburg. We get an impression of the atmosphere at the university at the time when we read Tietz's descriptions of his student days. Tietz relates that all the lecturers began their lecture with a Nazi salute and "Heil Hitler" (which was compulsory) except for Hecke who gave a silent nod to the class before beginning his lectures. Tietz writes about being with a group of fellow students when they met Hecke in the street (see  or  for slightly different versions):-
While coming from the Mathematics Department ... we overtook Professor Hecke. There was a bone-rattling chill in the air and we were all heavily wrapped up. With a smart "Heil Hitler, Herr Professor" and quickly raised right arm, my fellow students passed the old gentleman. With an astonished-indulgent look, Hecke glanced around him, raised his hat, lightly bowed, and said, "Good morning, ladies and gentlemen". After that I never heard the "Germanic greeting" from my fellow students.Tietz's permission to attend university because of his father's service in World War I only lasted a year. He was expelled from Hamburg University by the Nazis in December 1940 and his cousin Rolf Christiansen petitioned the Chancellor of the University in an attempt to allow him to be reinstated. The petition was unsuccessful and Christiansen, who was serving in the German army, was punished by being demoted and sent to the Russian front where he was killed in February 1942. Christiansen's mother took her own life on the day she learnt of her son's death. After Christiansen's petition failed, Tietz asked Erich Hecke what he should do and was told to continue to attend lectures secretly :-
Hecke said that naturally Tietz should come secretly to his mathematical lectures, also those of Hans Zassenhaus ... Tietz was somewhat afraid of Zassenhaus, who always wore the insignia of a Nazi organisation, but Hecke calmed him by saying that Zassenhaus was their trusted agent who behaved in a politically correct manner so as to protect them. This in fact happened in 1942, when somehow Tietz's presence as a secret student at the university became known and a denunciation was threatened: Zassenhaus warned Tietz, and Hecke immediately cancelled the class, giving back the student fees. In fact, Tietz only discovered much later that Zassenhaus was connected with a resistance effort that helped hide endangered people.In March 1942 Tietz reached the age of 21 and this allowed him to be registered as the new owner of his father's wood company. However, Tietz had no experience in that area at all and had never taken any interest in learning about this trade. Even before this the family realised the peril they were in since Tietz's aunt Dora, her husband Arnold and their daughter Anne Marie were deported to the ghetto of Minsk in November 1941. In July 1943 Hamburg was bombed, the Tietz home was destroyed and all their possessions were lost. Also the Tietz family were aware that the Gestapo were looking to arrest Willi Tietz. They decided to leave Hamburg and go to Marburg :-
... for no cogent reason other than that my parents had met each other here, and because my brother's friend, the future philosopher Gerd Guenther-Grey, was studying chemistry there.In a letter written to Tietz in Marburg, Erich Hecke suggested that he make contact with Kurt Reidemeister at the university. Reidemeister had been Hecke's student while studying for his doctorate. However, he was unable to make contact with Reidemeister before they were arrested by the Gestapo. In Marburg they lived in accommodation rented from a private landlady in the Upper Town. The rent had been set high by the landlady because she realised that the Tietz family were in danger of arrest. When they failed to keep up the high rent payments, the owner denounced the Tietz family as Jews to the Marburg Gestapo. On Christmas Eve in 1943 Tietz, his father and his mother, were all arrested by the Marburg Gestapo. Pressure was put on Amanda Tietz, who was Aryan, to sign papers divorcing her husband - she refused. Willi Tietz attempted to commit suicide believing this would save his wife but his attempt failed. Willi, Amanda and Horst were brought to the district court prison in the Wilhelmstrasse and kept there in individual cells until 6 March 1944 when they were sent to the work camp at Breitenau. This camp was being used as a collection point for those in the area who were to be sent on to concentration camps. At Breitenau, Tietz and his father were forced to undertake agricultural work.
Horst Tietz tried hard to take care of his father who became seriously ill. On 22 April 1944, Horst was allowed to join his father in a cell which was also occupied by eight other prisoners. On his way to this cell he saw a woman with a shaven head who was crying and shouting to him. It was his mother on her way to the transport to take her to the camp at Ravensbrück - he never saw her again. His father died in his arms in the morning of the next day and he later learnt that his mother died at Ravensbrück on 6 June. Tietz was transferred from Breitenau, first to a police cell in Leipzig, then on 2 May to Fürstenberg in Mecklenburg where the Ravensbrück concentration camp was situated, and finally to the Buchenwald concentration camp near Weimar. On 4 April 1945 the US 89th Infantry Division liberated a small subcamp of Buchenwald and on 8 April the Germans began to evacuate the prisoners from the main camp. Thousands were forced to join evacuation marches but Tietz was still in the camp when it was liberated by the US Third Army on 11 April. The soldiers were given a hero's welcome by the emaciated inmates. We note that, in the mid 1950s, Ludwig Bieberbach came to Tietz in tears saying that he had not known before about the true conditions in the concentration camps or about the Polish death camps.
Tietz returned to Marburg to try to restart his undergraduate studies but quickly moved to Hamburg. He wrote :-
I first made my way with difficulty to Marburg, but then to Hamburg, because there the university already started to function again on 6 November 1945. Erich Hecke, although he was mortally ill, lectured on Linear Differential Equations. Nowadays we cannot comprehend the situation - how hungrily the emaciated figures with their clothes in tatters followed the fascinating lecture in an atmosphere charged with tension. Hecke combined warmth with dignity, and thus revived an image of humanity that had become deformed during the Nazi era.At Hamburg he was able to attend courses and also take part in Hecke's seminar. He met Lotti who he later married. He learnt much from both Hecke and Zassenhaus but by the summer of 1946 Hecke had become too ill to lecture and Zassenhaus was preparing to emigrate. Tietz decided to return to Marburg :-
The war was over. In the four occupation zones life was stirring. Many universities opened their doors again. The Philipps University of Marburg began operating again in the year 1945-46. Because the city had been largely spared from the destruction of war, it exerted a strong attraction to streams of people returning home, refugees and the homeless who crisscrossed the country. The student body that expectantly filled the lecture rooms was accordingly very mixed: the students from Marburg, who still had the background of a family and had just come from school, were in stark contrast to these people, who were visibly in a terrible state.Kurt Reidemeister was the only Professor of Mathematics and he had the help of Associate Professor Maximilian Krafft (1893-1971). The other person, who only held an assistant's position, was Herbert Grötzsch (1902-1993). He had been a student of Paul Koebe and on the staff at Giessen. However, Giessen was not operating, so Grötzsch taught at Hamburg but was paid so little that he could neither feed nor clothe himself properly. However :-
Without Grötzsch mathematics teaching would have collapsed: he was tirelessly active and approachable.Tietz suffered serious health problems, particularly with his stomach and kidneys, which resulted from his time in the concentration camp. It was the care that he received from his wife Lotti that helped him recover his health during these years. We note that the Tietz family eventually became larger with the birth of two daughters. He took the State Examination in 1947. It required three subjects and he had chosen mathematics, physics and chemistry. However, his practical abilities in chemistry were very poor so when applied mathematics became a possible subject, he quickly changed to take the examination in that topic. He was examined in mathematics by Krafft and Grötzsch while in physics Erich Hückel was the examiner. After the examination Hückel asked Tietz if he would take a position as his assistant. Hückel said he understood the physics but thought that Tietz would be a good assistant to help him with the mathematics. Tietz held this position for three years while he worked for his doctorate advised by Maximilian Krafft. He was awarded the degree in 1950 by the Philipps-Universität Marburg for his thesis Fabersche Entwicklung auf geschlossenen Riemannschen Flächen . After his habilitation in Brunswick, in 1956 he was appointed as a lecturer in the University of Münster, where Heinrich Behnke had built a school of function theory. The other professor at Münster was Hans Petersson. Tietz taught at Münster until 1962 when he was appointed as professor of mathematics at the University of Hanover. From 1962 until his retirement in 1989, Tietz held this professorship, being Dean on several occasions.
We now look briefly at some of the papers that Tietz published before his habilitation. Die klassische Mechanik als Transformationstheorie (1951) gives a clever demonstration that Hamiltonian theory can be completely described by canonical transformations. In the same year he published Eine Rekursionsformel der Faberschen Polynome which presents a recursion formula for Faber polynomials. In Fabersche Entwicklungen auf geschlossenen Riemannschen Flächen (1952) Tietz describes a system of functions that play a similar role on a closed Riemann surface to that played by the Faber polynomials in plane regions. In Beweis der Konvergenz eines Verfahrens von W Bartky zur Berechnung von bestimmten Integralen (1952), Tietz discusses Walter Bartky's method for the evaluation of certain definite integrals. The 1953 paper Die Kinematik des starren Körpers was a joint work with Rudolf Iglisch in which the authors derive Euler's equation in kinematics by means of vector algebra. Tietz introduces a new aspect of the theory of conformal mapping in Zur Realisierung Riemannscher Flächen (1955). In Eine Normalform berandeter Riemannscher Flächen (1955) Tietz proves the theorem:
Any open Riemann surface of finite genus which has n non-degenerate boundary components can be mapped conformally onto a Riemann surface consisting of a number of full planes and n unit disks.We should also mention the textbooks that Tietz published. Lineare Geometrie (first edition 1967, second edition 1973) was a text based on lecture courses that Tietz had given on analytic geometry. The first chapter gives the geometrical background. In the second chapter the concepts of semigroup, group, ring and skew fields precede the definition of a left vector space. Then the standard results on linear dependence, bases, dimension etc. are given. The third chapter studies subspaces, direct sums, quotient spaces, dual spaces etc. The theory of linear transformations is discussed in Chapter 4 before studying a unified treatment of real and complex inner product spaces in Chapter 5. The final chapter looks at affine spaces.
He also contributed to a two-volume encyclopaedia of mathematics published in 1965, and the two-volume introduction to mathematics for engineers (1979-1980). The chapters are: 1. Figures and functions; 2. Linear geometry; 3. Elementary functions; 4. The analytical calculus; 5. Applications of analysis; 6. Series expansions; 7. Differential equations; 8. Multidimensional analysis.
Article by: J J O'Connor and E F Robertson