forming my research in adaptive control. Yakovs insistence on rigor in problem formulation andsolutions and his sharp analytical skills was a very good source of inspiration. Yakov also acceptedto receive my student Lennar Ljung for an extended visit in his laboratory. This strengthened theinteractions between our institutes. Yakov also arranged to have the books on Adaptive Controland Computer Control that I coauthored with Bjorn Wittenmark translated to Russian.

We also had many interesting social interactions during visits to the Soviet Union includingsome very memorable trips to Tashkent and Tblisi. It was very nice to have a guide who was verywell familiar with the eating and drinking habits.

There were formal agreements of exchange of scientist between Sweden and the Soviet Union.Tsypkin was invited many times to visit us in Sweden but somehow the visits did not materialize.The "rst time Yakov came to Lund was at the Second IFAC Workshop on Adaptive Systems inControl and Signal Processing in 1986. On this occasion, Yakov also had a special missionnamely to buy a computer for his grandson. Even though this was only 15 years ago, there wereseveral complications because of the export restrictions. It was at the time neither permissible toexport IBM PCs or machines with similar CPUs. After contacts with the appropriate authoritieswe found out that it was possible to export the Zinclair Z80. We were also lucky to "nd a storethat had these machines. A very nice coincidence was that another old friend Vaclav Peterka alsohad a similar machine with a lot of useful software. We discovered this by accident on a car tripacross Southern Sweden after the IFAC Workshop. In the end, we acquired a very nice packageand we put it on a rolling cart for Yakov to bring back home.

There are few persons who have in#uenced me as much as Yakov professionally. I am very sadthat he is no longer with us but I often think about him as a scientist and as a dear personal friend.His books and scienti"c contributions will be with us for a long time.

Some memories of Yakov Tsypkin

Shankar P. Bhattacharyya

Department of Electrical Engineering, Texas A & M University, College Station, TX 77845, U.S.A.

I "rst met Yakov Tsypkin when he attended an International Workshop on Robust Controlorganized by L.H. Keel and myself in March 1991 in San Antonio, Texas. After some inquiries (wehad to explain what Robust Control meant) the State Department had issued the Soviet scientistsTsypkin, Polyak and Kharitonov a 1 week visa to cover the duration of the Workshop. On theother hand, when news of their visit became known, invitations for lectures and visits poured infrom all over the U.S. I look them to the Immigration and Naturalization Service o$ce in SanAntonio to look into the possibility of extending their visa. They expressed the certainty that itwould be impossible. However, within 10 min of entering the building we had managed to extendtheir visa to a month. Tsypkin was overjoyed, he said &This is what the U.S.A. is all about*inRussia we would have never been able to do it*certainly not this easily'. We celebrated by havinglunch at the Hemisfair tower.

After the workshop, the three of them visited Texas A & M University. I requested Tsypkin toteach my undergraduate controls class and he agreed enthusiastically. During the 1 h lecture he

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taught them that the di!erence between the open- and closed-loop system is that the poles aredi!erent but the zeros are the same. He also visited the elementary school and Rice University wherehe lectured on the Tsypkin}Polyak locus for assessing the parametric stability margin. During visitsto my house he became very close to my children and insisted that I play Indian music for him.

I met him again the following year at Ascona, Switzerland, at a similar workshop. He told methat he would like to sit with me at lunch every day. When he came to know that we were writinga book, he immediately said that he would like to write a foreword for it. I was overwhelmed byhis child-like simplicity and directness. He was one of the greatest control scientists of the 20thcentury and I will personally cherish my brief association with him very dearly.

A personal remembrance of Tsypkin

Peter Dorato

University of New Mexico, EECE Department, Albuquerque, NI-87131, U.S.A.

I had known Prof. Tsypkin's for many years, mainly because of his work on relay andsampled-data systems, but in 1987 I had the special honor of being invited by him to give somelectures at the Institute of Control Science in Moscow. I lectured on robust control theory, basedlargely on the papers I had collected in my IEEE Reprint volume, &Robust Control'. He asked memany questions on the new robust theory that was being developed at the time. During my stay inMoscow, I received the further honor of being invited to dinner at his home. His wife prepareda wonderful Russian meal, and for the only time during my visit was I able to have some Vodka,since at the time there was a prohibition of serving Vodka in restaurants. In 1990, Tsypkin and hiswife visited Albuquerque, New Mexico. He presented a talk at the University of New Mexicoentitled, &Robust Stability of Linear Control Systems: A Frequency Response Approach'. Duringhis visit in New Mexico, I drove him and his wife to visit Acoma Pueblo. I had given him a copy ofa manuscript I had written, together with Fortuna and Muscato from the University of Catania,entitled, &Robust Control for Unstructured Perturbations*An Introduction', which he readduring our drive to Acoma. I am sure that his being a member of the Lecture Notes Advisory Boardof Springer-Verlag played an important role in Springer-Verlag's decision to publish the manu-script. When we arrived at the boundary of Acoma Pueblo, I remember how surprised Prof.Tsypkin was to "nd out we did not need any special visa to enter a &sovereign' Indian nation. So notonly will memories of his great works remain with me, but also memories of his great kindness.

Nine months with Professor Yakov Tsypkin

Kiatsukisa Furuta

Tokyo Denki University, Department of Computers and Systems Engineering Hatoyana Hikigun Saitana 350}0394, Japan

Professor Yakov Tsypkin and his wife Olga stayed in Japan from January to September 1993.Yakov Tsypkin was the Nippon Steel Professor, Chair of Intelligent Control, Department of

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