The Russian scientist, Academician Lev Vasil'yevich Ovsyannikov, who has made a major contribution to the development of mechanics and applied mathematics, was 80 years old on 22 April 1999. His classical results in the dynamics of a fluid with free boundaries and transonic gas dynamics are fundamental and the foundation of scientific theories which are being actively developed. Group analysis of differential equations, a fundamental scientific trend at the meeting point of mathematics and mechanics, in which he obtained a number of basic results, is today a powerful and universal instrument for investigating mathematical models in mechanics and physics. His scientific school is well known in world science.
A detailed description of his scientific work can be obtained from the publications associated with his 70th birthday [1,2]. In this paper, we therefore wish to give a more detailed account of some notable events in his life and scientific activity in the 1990s.
Lev Vasil'yevich Ovsyannikov was born in the town of Vasil'sursk on the Volga. In 1937, he entered the mechanics--mathematics faculty of Moscow University but his studies were interrupted by the war. In the first months of the war, he, together with other students at Moscow State University, helped to construct the defences around Moscow. In the autumn of 1941, he finished his university studies and became a student at the Leningrad Air Force Engineering Academy. After the end of the war in 1945, he became an advanced student at the Academy and subsequently, up to 1953, lectured there and at Leningrad University. His outstanding scientific results in the theory of transonic gas flows date from this period and his candidate dissertation, which he defended in 1949, dealt with these results. We shall mention just a few of them. He discovered for the first time and investigated in detail the special features of gas flow at a straight sonic line and developed a method for analysing such flows. A result which is remarkable in its beauty and importance is due to him: when there is an outflow of a subsonic gas jet with a critical velocity at a boundary, the flow becomes uniform at a finite distance from the aperture. This fact, which became known in transonic gas dynamics as Ovsyannikov's theorem, was one of the first results of a scientific method which was formulated later, namely, the localization of the solutions of degenerate non-linear differential equations.