Mathematics in life and researches of L.G. Loytsyansky. To the 120th anniversary of his birth

Fragments of the history of the Saint-Petersburg (Leningrad) Polytechnic Institute, founded in 1899, are stated with the emphasis on the development of the mechanical and mathematical components of the Institute in the first half of the 20th century. Some information on the course of life of Professor Lev Gerasimovich Loitsyansky is given; in particular, it is touched upon his “zigzag” way of pursuing high mathematical education during the revolution and civil-war years. A gallery of mathematical methods used in the classical, reissued many times, textbooks by Loitsyansky in the field of theoretical mechanics and fluid dynamics is presented. Short descriptions of mathematical solutions got by Loitsyansky for several generic problems in the viscous fluid dynamics are given. The prehistory and the development of ideas, which have resulted in the development by Loitsyansky (in the sixties) a unique generalized-similarity method for solving nonlinear problems in the boundary layer theory are described as well.