In mathematics, the Lyusternik–Schnirelmann category (or, Lusternik–Schnirelmann category, LS-category) of a topological space formula_1 is the homotopical invariant defined to be the smallest integer number formula_2 such that there is an open covering formula_3 of formula_1 with the property that each inclusion map formula_5 is nullhomotopic.
 For example, if formula_1 is a sphere, this takes the value two.
Yuri Vladimirovich Linnik (Russian: Ю́рий Влади́мирович Ли́нник ; January 8, 1915 – June 30, 1972) was a Soviet mathematician active in number theory, probability theory and mathematical statistics.
Hà Huy Khoái (Ha Tinh, 24 November 1946) is a Vietnamese mathematician.
 Hà Huy Khoái studied in Vietnam under the "fathers" of Vietnamese mathematics Lê Văn Thiêm and Hoàng Tụy, and in Moscow at the Steklov Institute of Mathematics under Yuri I. Manin.
 His main field of work has been p-adic Nevanlinna theory, for example proving part of a non-Archimedean version of Green's theorem (AMS, 1992, 503-509).
Lev Genrikhovich Schnirelmann (also Shnirelman, Shnirel'man; "Лев Ге́нрихович Шнирельма́н" ; January 2, 1905 – September 24, 1938) was a Soviet mathematician who worked on number theory, topology and differential geometry.
The inscribed square problem, also known as the square peg problem or the Toeplitz' conjecture, is an unsolved question in geometry: "Does every plane simple closed curve contain all four vertices of some square?"
 This is true if the curve is convex or piecewise smooth and in other special cases.
 The problem was proposed by Otto Toeplitz in 1911.
 Some early positive results were obtained by Arnold Emch and Lev Schnirelmann.
 s of 2016 the general case remains open.
In mathematics, the Lusternik–Schnirelmann theorem, aka Lusternik–Schnirelmann–Borsuk theorem or LSB theorem, says as follows.
David Wilson Henderson is a Professor Emeritus of Mathematics in the Department of Mathematics at Cornell University.
 His work ranges from the study of algebraic geometry, Persian history of mathematics and exploratory mathematics for teaching prospective mathematics teachers.
 His papers in the philosophy of mathematics place him with the intuitionist school of philosophy of mathematics.
 His practical geometry, which he puts at work and discovers in his carpentry work, gives a perspective of geometry as the understanding of the infinite spaces through local properties.
 Euclidean geometry is seen in his work as extendable do the spherical and hyperbolic spaces starting with the study and reformulation of the 5th postulate
Yuri Ivanovich Zhuravlyov (Russian: Юрий Иванович Журавлёв ; born 14 January 1935 in Voronezh) is a Russian mathematician specializing in the algebraic theory of algorithms.
 His research in applied mathematics and computer science has become foundational for a number of specialties within discrete mathematics, pattern recognition, and predictive analysis.
 Zhuravlev is a full member of the Russian Academy of Sciences and the chairman of its "Applied Mathematics and Informatics" section.
 He is also the Editor-in-Chief of the international journal "Pattern Recognition and Image Analysis".
Applied mathematics is a branch of mathematics that deals with mathematical methods that find use in science, engineering, business, computer science, and industry.
 Thus, applied mathematics is a combination of mathematical science and specialized knowledge.
 The term "applied mathematics" also describes the professional specialty in which mathematicians work on practical problems by formulating and studying mathematical models.
 In the past, practical applications have motivated the development of mathematical theories, which then became the subject of study in pure mathematics where abstract concepts are studied for their own sake.
 The activity of applied mathematics is thus intimately connected with research in pure mathematics.
Lev Efimovich Kerbel (Russian: Лев Ефимович Кербель ; November 7 [O.S. October 25] 1917 - 14 August 2003) was a sculptor of Soviet realist works.
 Kerbel's creations included statues of Marx, Lenin, Yuri Gagarin, which were sent by Soviet Government as gifts to socialist and the Third World countries across the world.
 In 1976 the Council of Ministers of the Soviet Union presented the Government of Sri Lanka the monument of Solomon Bandaranaike, the late Prime minister of the country, carved by Lev Kerbel.

Question: What area of mathematics did both Lev Schnirelmann and Yuri Linnik work on?
Answer: number theory