Lipót Fejér

Lipót Fejér (Hungarian: Fejér Lipót; originally Leopold Weiss; 9 February 1880 – 15 October 1959) was a Hungarian mathematician of Jewish descent renowned for his foundational contributions to harmonic analysis and the theory of Fourier series. At the age of 20, he proved that the Cesàro means of the partial sums of the Fourier series of a continuous function converge uniformly to the function itself, a result known as Fejér's theorem, which resolved longstanding issues regarding the convergence properties of these series. Fejér's early name change from Weiss to Fejér reflected a common practice among Hungarian Jews to adopt more nationalistic surnames around 1900. Appointed professor of mathematics at the University of Budapest (now Eötvös Loránd University) in 1905 as successor to Manuel Beke, he mentored a generation of leading mathematicians, including John von Neumann, Paul Erdős, George Pólya, and Pál Turán, supervising over 20 doctoral students whose intellectual descendants number in the thousands. His collaborations, notably with Constantin Carathéodory on complex analysis and potential theory, further advanced approximation theory and its applications. Fejér's emphasis on constructive methods and his prolific output, spanning over 600 papers, established him as a pivotal figure in the golden age of Hungarian mathematics.

He was born into a Jewish family of modest means, with his father Samu (or Samuel) Weiss operating a local shop and his mother Viktória Goldberger managing the household. Originally surnamed Weiss—a German term meaning "white"—Fejér adopted the Hungarian equivalent Fejér, signifying "steam" but also connoting whiteness, during his school years as part of a broader assimilation trend among Hungarian Jews. His family's background reflected the socioeconomic conditions of provincial Jewish merchants in late 19th-century Hungary, where commercial occupations were common amid limited opportunities for higher education or professions.

Fejér, born Leopold Weisz on February 9, 1880, in Pécs, Hungary, to a Jewish shopkeeper father named Samu Weiss, began his formal education in local primary schools. Early performance was poor, particularly in mathematics, prompting his father to withdraw him from school temporarily and provide home instruction himself.

Lipót Fejér was born Leopold (or Lipót) Weisz on 9 February 1880 in Pécs, then part of Austria-Hungary, into a Jewish family. His father, Samuel (Samu) Weiss, managed a local shop, while his mother was Viktória Goldberger; the family's Jewish heritage traced back through maternal lines, including a great-grandfather, Samuel Nachod, who had been a physician.