Sergiy Dmytrovych  Sinel’shchikov (1960-2020)

 Sergiy Dmytrovych  Sinel’shchikov was born on 15 November, 1960 in Kharkiv. His father was a well-regarded engineer on Turboatom factory, and his mother was a medical practitioners.  In 1967, Sergiy entered school 61 (Kharkiv), which he graduated with a gold medal in 1977. In the same year he entered the Faculty of Mechanics and Mathematics of Kharkiv University (V.N. Karazin Kharkiv National University now). In his third year of university, he started to study ergodic theory  under the guidance of V.Ya. Golodets. After graduation from university in 1982, he was accepted into graduate school at the Department of Algebra and Function Theory of Kharkiv University. Sergiy was chairman of the student scientific society for a while. His first results are related to the structure of the cocycles of ergodic equivance relations with values in a locally compact separable group. In particular, Sergiy (joint with V.Ya. Golodets) proved that every solvable group is an image of the cocycle of an ergodic automorphism. This result formed the basis of his Ph.D. thesis, which he defended in 1986. From 1986 to 1989 he worked as a junior researcher at the Department of Mathematical Physics of Kharkiv University. At the same time Sergiy taught algebra at the Faculty of Mechanics and Mathematics. Over thirty years, since 1989, Sergiy Dmytrovych  worked at B. Verkin Institute for Low Temperatures Physics and Engineering of the National Academy of Sciences of Ukraine (Department of Mathematical Physics (1989-2017) and Department of Function Theory (2017-2020)).

Initially, his research interests were in ergodic theory. Sergiy collaborated with world top specialists in this field: V.Ya.Golodets, D.Rudolph, M.Leman’czyk, A.Dooley, J.P.Thouvenot and others. In joint works with them, he obtained a number of profound results that are highly appreciated in the community of specialists in ergodic theory and related fields. These advances include classification of cocycles of hyperfinite equivalence relations and some pioneering results on isomorphism problem for the stabilizers of non-free ergodic group actions. Several works were devoted to extension of the classical entropy theory to amenable group actions: construction of non-Bernoulli systems with completely positive entropy, description of the Pinsker algebras, etc.

Under an influence of L.L.Vaksman (especially after V.Ya Golodets left Kharkiv) Sergiy’s research interests gradually moved to the quantum group theory. He studied Quantum Bounded Symmetric Domains and actions of quantum groups and obtain many remarkable results. Among them is the full description of symmetries of the quantum plane and its Loran extension. Jointly with L.Vaksman, Segiy wrote 14 research papers. He also collaborated actively with A.Stolin, S.Duplij, O.Berstein in this field. After Vaksman’s untimely death, Sergiy supervised Kolisnyk, who was a doctorate student of Vaksman, and helped him to prepare the PhD thesis for defense. Sergiy also completed Vaksman’s book ‘‘Quantum Bounded Symmetric Domains’’ and submitted it to the publisher.

In 2016, Sergiy Sinelshchikov defends his doctoral dissertation  “The actions of non-abelian groups and quantum algebras on the point spaces and their analogous” . The dissertation covers his results from both the entropy theory and the quantum group theory. Sergiy Dmytrovych  Sinel’shchikov’s colleagues will always remember his readiness to help in any situation.