Czesław Ryll-Nardzewski (; 7 October 1926 – 18 September 2015) was a Polish

mathematician A mathematician is someone who uses an extensive knowledge of mathematics in their work, typically to solve mathematical problems. Mathematicians are concerned with numbers, data, quantity, structure, space, models, and change. History On ...

. Born in

Wilno Vilnius ( , ; see also #Etymology and other names, other names) is the capital and List of cities in Lithuania#Cities, largest city of Lithuania, with a population of 592,389 (according to the state register) or 625,107 (according to the munic ...

Second Polish Republic The Second Polish Republic, at the time officially known as the Republic of Poland, was a country in Central Europe, Central and Eastern Europe that existed between 1918 and 1939. The state was established on 6 November 1918, before the end of ...

(now Vilnius,

Lithuania Lithuania (; lt, Lietuva ), officially the Republic of Lithuania ( lt, Lietuvos Respublika, links=no ), is a country in the Baltic region of Europe. It is one of three Baltic states and lies on the eastern shore of the Baltic Sea. Lithuania ...

), he was a student of

Hugo Steinhaus Hugo Dyonizy Steinhaus ( ; ; January 14, 1887 – February 25, 1972) was a Polish mathematician and educator. Steinhaus obtained his PhD under David Hilbert at Göttingen University in 1911 and later became a professor at the Jan Kazimierz Unive ...

. At the age of 26 he became professor at

Warsaw University The University of Warsaw ( pl, Uniwersytet Warszawski, la, Universitas Varsoviensis) is a public university in Warsaw, Poland. Established in 1816, it is the largest institution of higher learning in the country offering 37 different fields of ...

. In 1959, he became a professor at the

Wrocław University of Technology Wrocław (; german: Breslau, or . ; Silesian German: ''Brassel'') is a city in southwestern Poland and the largest city in the historical region of Silesia. It lies on the banks of the River Oder in the Silesian Lowlands of Central Europe, rou ...

. He was the advisor of 18 PhD theses. His main research areas are

measure theory In mathematics, the concept of a measure is a generalization and formalization of geometrical measures ( length, area, volume) and other common notions, such as mass and probability of events. These seemingly distinct concepts have many simil ...

functional analysis Functional analysis is a branch of mathematical analysis, the core of which is formed by the study of vector spaces endowed with some kind of limit-related structure (e.g. Inner product space#Definition, inner product, Norm (mathematics)#Defini ...

foundations of mathematics Foundations of mathematics is the study of the philosophy, philosophical and logical and/or algorithmic basis of mathematics, or, in a broader sense, the mathematical investigation of what underlies the philosophical theories concerning the natu ...

and

probability theory Probability theory is the branch of mathematics concerned with probability. Although there are several different probability interpretations, probability theory treats the concept in a rigorous mathematical manner by expressing it through a set o ...

. Several theorems bear his name: the

Ryll-Nardzewski fixed point theorem In functional analysis, a branch of mathematics, the Ryll-Nardzewski fixed-point theorem states that if E is a normed vector space and K is a nonempty convex subset of E that is compact under the weak topology, then every group (or equivalently: eve ...

, “9. Theorem of Ryll-Nardzewski” (p. 171), “(9.6) Theorem (Ryll-Nardzewski)” (p. 174) the Ryll-Nardzewski theorem See Theorem 7.3.1 Cf. (2.10) in

model theory In mathematical logic, model theory is the study of the relationship between formal theories (a collection of sentences in a formal language expressing statements about a mathematical structure), and their models (those structures in which the s ...

, and the

Kuratowski and Ryll-Nardzewski measurable selection theorem In mathematics, the Kuratowski–Ryll-Nardzewski measurable selection theorem is a result from measure theory that gives a sufficient condition for a set-valued function to have a measurable selection function. It is named after the Polish mathem ...

. See Theorem 6.9.3 on p. 36 and the historical comment on p. 441 He became a member of the

Polish Academy of Sciences The Polish Academy of Sciences ( pl, Polska Akademia Nauk, PAN) is a Polish state-sponsored institution of higher learning. Headquartered in Warsaw, it is responsible for spearheading the development of science across the country by a society of ...

in 1967. He died in 2015 at the age of 88Zmarł Profesor Czesław Ryll-Nardzewski
and he is buried in the Wrocław, cm. Grabiszyński.

References

Short biography
at the Polish Academy of Sciences 1926 births 2015 deaths Model theorists Scientists from Vilnius People from Wilno Voivodeship (1926–1939) Polish mathematicians Recipients of the State Award Badge (Poland) {{Poland-mathematician-stub