Stefan Banach (March 30, 1892 – August 31, 1945) was a Polish mathematician. He is generally considered to have been one of the 20th century's most important and influential mathematicians. Banach was one of the founders of modern functional analysis and one of the original members of the Lwów School of Mathematics. His major work was the 1932 book, Théorie des opérations linéaires (Theory of Linear Operations), the first monograph on the general theory of functional analysis.

Born in Kraków, Banach enrolled in the Henryk Sienkiewicz Gymnasium, a secondary school, and worked on mathematics problems with his friend Witold Wiłkosz. After graduating in 1910, Banach and Wiłkosz moved to Lwów. However, Banach returned to Kraków during World War I, and during this time he met and befriended Hugo Steinhaus. After Banach solved mathematical problems which Steinhaus considered difficult, he and Steinhaus published their first joint work. Along with several other mathematicians, Banach formed a society for mathematicians in 1919. In 1920, after Poland had in 1918 regained independence, Banach was given an assistantship at Jagiellonian University. He soon became a professor at the Lwów Polytechnic and a member of the Polish Academy of Learning. Later Banach organized the "Lwów School of Mathematics". Around 1929 he began writing Théorie des opérations linéaires.

After the outbreak of World War II, in September 1939, Lwów was taken over by the Soviet Union. Banach became a member of the Academy of Sciences of Ukraine and was the dean of the Department of Mathematics of Physics of Lwów University. In 1941, when Germans took over the city, all institutions of higher education were closed to Poles. As a result, Banach had to earn money as a feeder of lice at Rudolf Weigl's Institute for Study of Typhus and Virology. While the job carried the risk of becoming infected with typhus, it protected him from being sent to slave labor in Germany and other forms of repression. When the Soviets recaptured Lwów in 1944, Banach reestablished the University. However, because the Soviets were removing Poles from annexed formerly Polish territories, Banach prepared to return to Kraków. He died in August 1945 after being diagnosed with lung cancer seven months earlier.

Some of the notable mathematical concepts named after Banach include Banach spaces, Banach algebras, the Banach–Tarski paradox, the Hahn–Banach theorem, the Banach–Steinhaus theorem, the Banach-Mazur game, the Banach–Alaoglu theorem and the Banach fixed-point theorem.

Early life

Stefan Banach was born on 30 March 1892 at St. Lazarus General Hospital in Kraków, then part of the Austro-Hungarian Empire. Banach's parents were Stefan Greczek and Katarzyna Banach, both natives of the Podhale region. Greczek was a soldier in the Austro-Hungarian Army stationed in Kraków. Little is known about Banach's mother.

Unusually, Stefan's surname was that of his mother instead of his father, though he received his father's given name, Stefan. Since Stefan Greczek was a private and was prevented by military regulations from marrying, and the mother was too poor to support the child, the couple decided that he should be reared by family and friends. Stefan spent the first few years of his life with his grandmother, but when she took ill Greczek arranged for his son to be raised by Franciszka Płowa and her niece Maria Puchalska in Kraków. Young Stefan would regard Franciszka as his foster mother and Maria as his older sister. In his early years Banach was tutored by Juliusz Mien, a French intellectual and friend of the Płowa family, who had emigrated to Poland and supported himself with photography and translations of Polish literature into French. Mien taught Banach French and most likely encouraged him in his early mathematical pursuits.

In 1902 Banach, aged 10, enrolled in Kraków's Henryk Sienkiewicz Gymnasium (also known as the Goetz Gymnasium). While the school specialized in the humanities, Banach and his best friend Witold Wiłkosz (also a future mathematician) spent most of their time working on mathematics problems during breaks and after school. Later in life Banach would credit Dr. Kamil Kraft, the mathematics and physics teacher at the gymnasium with kindling his interests in mathematics. While generally Banach was a diligent student he did on occasion receive low grades (he failed Greek during his first semester at the gymnasium) and would later speak critically of the school's math teachers.

After obtaining his matura (high school degree) at age 18 in 1910 Banach, together with Wiłkosz, moved to Lwów with the intention of studying at the Lwów Polytechnic. He initially chose engineering as his field of study since at the time he was convinced that there was nothing new to discover in mathematics. At some point he also attended Jagiellonian University in Kraków on a part-time basis. As Banach had to earn money to support his studies it was not until 1914 that he finally, at age 22, passed his high school graduation exams.

When World War I broke out, Banach was excused from military service due to his left-handedness and poor vision. When the Russian Army opened its offensive toward Lwów, Banach left for Kraków, where he spent the rest of the war. He made his living as a tutor at the local gymnasiums, worked in a bookstore and as a foreman of road building crew. He may have attended lectures at the Jagiellonian University at that time, including those of the famous Polish mathematician Stanisław Zaremba, but little is known of that period of his life.

Discovery by Steinhaus

In 1916, in Kraków's Planty gardens, Banach encountered Professor Hugo Steinhaus, one of the renowned mathematicians of the time. According to Steinhaus, while he was strolling through the gardens he was surprised to overhear the term "Lebesgue measure" (Lebesgue integration was at the time still a fairly new idea in mathematics) and walked over to investigate. As a result he met Banach, as well as Otto Nikodym and Wilkosz. Steinhaus became fascinated with the self-taught young mathematician. The encounter resulted in a long-lasting collaboration and friendship. In fact, soon after the encounter Steinhaus invited Banach to solve some problems he had been working on but which had proven difficult. Banach solved them within a week and the two soon published their first joint work (On the Mean Convergence of Fourier Series). Steinhaus, Banach and Nikodym, along with several other Kraków mathematicians (Władysław Ślebodziński, Leon Chwistek, Jan Kroć, and Włodzimierz Stożek) also established a mathematical society, which eventually became the Polish Mathematical Society. The society was officially founded on April 2, 1919. It was also through Steinhaus that Banach met his future wife, Łucja Braus.

Interbellum

Steinhaus introduced Banach to academic circles and substantially accelerated his career. After Poland regained independence, in 1920 Banach was given an assistantship at Kraków's Jagiellonian University. Steinhaus' backing also allowed him to receive a doctorate without actually graduating from a university. The doctoral thesis, accepted by King John II Casimir University of Lwów in 1920  and published in 1922, included the basic ideas of functional analysis, which was soon to become an entirely new branch of mathematics. The thesis was widely discussed in academic circles and allowed him in 1922 to become a professor at the Lwów Polytechnic. Initially an assistant to Professor Antoni Łomnicki, in 1927 Banach received his own chair. In 1924 he was also accepted as a member of the Polish Academy of Learning. At the same time, from 1922, Banach also headed the second Chair of Mathematics at University of Lwów.

Young and talented, Banach gathered around him a large group of mathematicians. The group, meeting in the Scottish Café, soon gave birth to the "Lwów School of Mathematics". In 1929 the group began publishing its own journal, Studia Mathematica, devoted primarily to Banach's field of study — functional analysis. Around that time, Banach also began working on his best-known work, the first monograph on the general theory of linear-metric space. First published in Polish in 1931, the following year it was also translated into French and gained wider recognition in European academic circles. The book was also the first in a long series of mathematics monographs edited by Banach and his circle.

World War II

Following the invasion of Poland by Nazi Germany and the Soviet Union, Lwów came under the control of the Soviet Union for almost two years. Banach, from 1939 a corresponding member of the Academy of Sciences of Ukraine, and on good terms with Soviet mathematicians, had to promise to learn Ukrainian to be allowed to keep his chair and continue his academic activities. Following the German takeover of Lwów in 1941 during Operation Barbarossa, all universities were closed and Banach, along with many colleagues and his son, was employed as lice feeder at Professor Rudolf Weigl's Typhus Research Institute. Employment in Weigl's Institute provided many unemployed university professors and their associates protection from random arrest and deportation to Nazi concentration camps.

After the Red Army recaptured Lviv in the Lvov–Sandomierz Offensive of 1944, Banach returned to the University and helped re-establish it after the war years. However, because the Soviets were removing Poles from annexed formerly Polish territories, Banach began preparing to leave the city and settle in Kraków, Poland, where he had been promised a chair at the Jagiellonian University. He was also considered a candidate for Minister of Education of Poland. In January 1945, however, he was diagnosed with lung cancer and was allowed to stay in Lwów. He died on August 31, 1945, aged 53. His funeral at the Lychakiv Cemetery was attended by hundreds of people.