[1][2][3] He is also known as the author of classical books on various subjects in analysis, and for his work on the history of mathematics.

In 1928, he defended his PhD thesis "Aerodynamical Investigations" under the supervision of Dmitry Grave.

From 1933 to his death, except for the years of war and evacuation, he was a professor at Kharkov University and at other institutes in Kharkiv.

[2][3] He found the fundamental connection between the inverse problem for important classes of differential and finite difference operators of the second order with a finite number of gaps in the spectrum, and the Jacobi inversion problem for Abelian integrals.

[3] This connection led to explicit solutions of the inverse problem for the so-called finite-gap operators.