After graduating from the Kishinev University, K.S. Sibirsky was Professor at the same University and Chairman of Department of Differential Equations of the Institute of Mathematics, AS of Moldova. He was a skillful teacher, devoted mentor and talented researcher. He lavishly shared his experience, knowledge and enthusiasm with his disciples thus creating a friendly cooperative atmosphere.

The scientific activity of K.S. Sibirsky was mainly devoted to the study of differential equations and dynamical systems. He made significant contributions to the qualitative theory of polynomial differential equations in the plane. He obtained necessary and sufficient conditions for the existence of a center of a quadratic system and investigated algebraic particular integrals. K.S. Sibirsky proposed and developed a new fruitful approach to the study of polynomial differential systems, the method of algebraic invariants and comitants (with respect to the group of linear transformations of coefficients induced by linear maps of the plane). By using this method, Konstantin Sergeevich was able to solve a number of problems concerning affine and topological classification of polynomial vector fields. These investigations were widely acknowledged by specialists and continued by many of his disciples. K.S. Sibirsky was also interested in topological dynamics and obtained many results in the theory of (semi)dynamical systems without uniqueness. In all, he published about 140 scientific papers, including 5 monographs. His results were reported many times in international conferences and workshops.

K.S. Sibirsky was a member of the editorial board of the journal Differential Equations and deputy editor-in-chief of the Bulletin of the Academy of Sciences of Moldova Mathematics.

K.S. Sibirsky contributed a lot to the development of mathematics in Moldova. He founded a well-known scientific school in differential equations. In 1960, he organized the Kishinev seminar on the qualitative theory of differential equations. He supervised 15 Ph.D. theses.

K.S. Sibirsky was a member of the Bureau of the Department of Physico technical and Mathematical Sciences, AS of Moldova, deputy chairman of the Council for defense of Ph.D. theses, chairman of the Scientific Council of Moldova on Theoretical Mathematics. He was awarded the title Honorary Researcher of Moldova, the State Prize of Moldova and other decorations.

Konstantin Sergeevich was a man of generous spirit. His devotion to science, diligence, humanity, care for his students and colleagues have won enormous respect of all who knew him. He lives on in the minds of his disciples.

B.A. Shcherbakov, I.U. Bronshteyn

**Konstantin Sergeevich Sibirsky (1928 - 1990). A tribute in honor of his 70th birthday.**- Hilbert's series for comitants graded algebras of differential equations. (English)
- Smooth linearization near a three-dimensional saddle. (English)
- Recurrent functions and recurrent solutions of differential equations. (Russian)
- Lyapunov functions for cocycle attractors in nonautonomous difference equations. (English)
- Attractors for stochastic differential equations with nontrivial noise. (English)
- On the law of transformation of affine connection and its integration. Part 1. Generalization of the Lame equations. (English)
- Types of the critical points for quadratic system with a center of symmetry. (English)
- The differential operators and multiplicity of singular points for polynomial differential system. (English)
- Poisson stability of mappings with respect to a semigroup. (English)
- On partial recurrent solutions of completely integrable systems. (English)
- Coefficient conditions for the repartition of multiplicity between 2 real double infinite singular points of cubic differential system. (English)
- $L_p$-theory of generalized solutions of model boundary value problems for parabolic equations. (English)
- Local phase portraits of two-dimensional analytical systems principal parts of which do not contain parameters. (English)