It seemed that the text of the paper had already been shaped in Ya.Z.'s mind; de"nitely, this impression was wrong*Ya.Z. revised his manuscripts many times. He always returned back from vacations with a notepad full of comments, calculations, and questions. We sat and discussed them for hours. He was the most disciplined attendee at any conference, even if the subject of a lecture was beyond the "eld of his direct interests and a presentation was far from being perfect. Tsypkin's questions were aimed at the very core of the topic, and they often clari"ed the situation for the lecturer himself.
Tsypkin had numerous pupils*he supervised more than 50 PhD theses, and many of his former students became well-known researchers. Many other scientists, who were not his students formally, refer to Tsypkin as their teacher. His style of teaching was non-standard; it was based on the idea &To teach one swim*drop the guy into water.' He just involved a student into research without a detailed explanation. Usually such an approach happened to be successful; the student worked hard to understand everything himself and, step-by-step, became an expert in the problem. The trick was that the problem itself was deep enough and interesting.
Ya.Z. had quite a speci"c style of absorbing new knowledge. He read a lot*all available journals in control as well as various books in mathematics and physics. He did not miss proceedings of numerous conferences and workshops. A new book was the best gift for him. He collected a huge library, and it is a great value nowadays, in the situation of shortage of books in Russia. Ya.Z. often found a source of new ideas and links in the remote "elds of knowledge. New ideas attracted him completely, and he often changed directions of his research. Thus, he switched from delay systems to pulse systems, then to relay systems; later he dealt with adaptation and learning, robust and optimal identi"cation; the latest areas of his interest were robust stability and robust control. He had broad understanding of control theory as extremely rich and diversi"ed; at the same time he understood it as something uni"ed. In his opinion, the frequencydomain theory of linear systems was the heart of this science. When he learnt about the Nyquist test at his student years, he loved it forever, he was saying.
Research was the best medication that cured him of inavoidable troubles of the real-life world, which was not so friendly in this country. After unpleasant events such as tensions with the institute authorities or results of the Academy elections, he used to say: &&Boris, let's talk about our paper under preparation, to make my mood up.'' But actually, quite seldom was he under such oppressed emotions; Ya.Z. was a lucky person, and he was in a balance with his environment. He had no enemies, and I never heard him speaking badly about anybody. Just a few days before he passed away he said that if to summarize all his long life, he evaluates it as a happy one.
That day, December 2, 1997, we had a seminar at our lab. Tsypkin came to the Institute, but suddenly fell ill. We called for ambulance; before entering it, Ya.Z. tried to smile and said: &&Well, have the seminar without me.'' He died a few hours later 2