# Egorov, Dimitry Fedorovich

# Egorov, Dimitry Fedorovich

(*b*. Moscow, Russia, 22 December 1869; *d*. Kazan, U.S.S.R., 10 September 1931)

*mathematics*.

After graduating from the Gymnasium in Moscow, Egorov entered the division of physics and mathematics of Moscow University, from which he received a diploma in 1891. After obtaining his master’s degree he remained at the university to prepare for a professorship, and in 1894 he became *Privatdozent* there. In 1901 he received his doctorate with a dissertation entitled “Ob odnom klasse ortogonalnykh sistem,” and two years later he was appointed extraordinary professor in the division of physics and mathematics at the University of Moscow. In 1909 Egorov was made ordinary professor and was appointed director of the Mathematical Scientific Research Institute. He was elected corresponding member of the USSR Academy titled “Ob odnom klasse ortogonalnykh sistem,” and member. Egorov was a member of the Moscow Mathematical Society; in 1902 he was elected to the French Mathematical Society and the Mathematical Society of Berlin University. From 1922 almost until his death, he was president of the Moscow Mathematical Society, and from 1922 he was editor-in-chief of *Matematicheskii sbornik*.

Egorov’s investigations on triply orthogonal systems and potential surfaces, i.e., surfaces *E*, contributed greatly to differential geometry. The results of these investigations were presented by Darboux in his monograph *Leçons sur les systèmes orthogonaux et les coordonnées curvilignes* (2nd ed., Paris, 1910).

Egorov considerably advanced the solution of Peterson’s problem on the bending on the principal basis. In the theory of functions of a real variable, wide use is made of Egorov’s theorem: Any almost-everywhere converging sequence of measurable functions converges uniformly on a closed set, the complement of which has an infinitely small measure. This theorem, as well as Egorov’s scholarship in new trends, led to the creation of the Moscow school dealing with the theory of functions of a real variable. Among the mathematicians belonging to Egorov’s school are the well-known Soviet mathematicians N. N. Lusin, V. V. Golubev, and V. V. Stepanov.

Egorov also worked in other areas; for instance, he initiated an investigation into the theory of integral equations. A brilliant lecturer and scholar, Egorov wrote some college textbooks on the theory of numbers, on the calculus of variations, and on differential geometry.

## BIBLIOGRAPHY

Egorov’s writings include “Uravnenia s chastnymi proisvodnymi vtorogo poriadka po dvum nezavisimym peremenym,” in *Uchenye zapiski Moskovskogo universiteta*, **15** (1899), i–xix, 1–392; “Ob odnom klasse ortogonalnykh sistem,” *ibid.*, **18** (1901), i–vi, 1–239, his doctoral diss.; “Ob izgibanii na glavnom osnovanii pri odnom semeystve ploskikh ili konicheskikh linii,” in *Matematicheskii sbornik*, **28** (1911), 167–187; “Sur les suites de fonctions mesurables,” in *Comptes rendus hebdomadaires des séances de l’Académie des sciences*, **152** (1911), 244–246; and “Sur l’intégration des fonctions mesurables,” *ibid.*, **155** (1912), 1474–1475; *Elementy teorii chisel* (Moscow, 1923); *Differentsialnaya geometria* (Moscow-Petrograd, 1924); “Sur les surfaces, engendrées par la distribution des lignes d’une famille donnée,” in *Matematicheskii sbornik*, **31** (1924), 153–184; “Sur la théorie des équations intégrales au noyau symétrique,” *ibid.*, **35** (1928). 293–310; “Sur quelques points de la théorie des équations intégrales à limites fixes.” in *Comptes rendus hebdomadaires des séances de l’Académie des sciences*, **186** (1928), 1703–1705.

A secondary source is V. Steklov, P. Lazarev, and A. Belopolsky, “Zapiska ob uchenykh trudakh D. F. Egorova,” in *Izvestiya Rossiiskoi akademii nauk*, **18** , no. 12–18 (1924), 445–446.

A. B. Paplauskas

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**Egorov, Dimitry Fedorovich**