Commemorative: Mathematician Yuri Kondratiev (1953-2023)

Funchal, Madeira, 2009. Yuri with his PhD students: Top (left to right): Tobias Kuna, Oles Kutoviy, Martin Grothaus. Bottom (left to right): Jose Luis da Silva, Dmitri Finkelshtein, Maria Joao Oliveira, Yuri Kondratiev. (Private)

On September 5, 2023, a sad news reached us: one of the founding members of the German-Ukrainian Academic Society Professor Yuri Kondratiev, passed away. Yuri was not only pationate about his own research, but also a strong promotor of international, and in particular, German-Ukrainian scientific cooperation. We express our deep condolences to his family, his students and colleagues.

Here we give the word to those, who knew Yuri as a colleague and as a teacher:

Prof. Mykola Leonenko, Cardiff University, UK

Yuri Kondratiev was a brilliant member of the Kyiv school of functional analysis. We met back in university years, and then for many years we discussed mainly scientific problems on many conferences and workshops. Yuri’s  research interests included functional analysis, mathematical physics and stochastics, especially limit theorems and related continuous time random walks, large deviation theory, Markov processes and diffusions, including non-local diffusions. All his life, Yuri Kondratiev leisurely wandered in the garden of converging paths, which always led him to the right place (“Kondratiev space”) at the right time.

Rome, 2014 (left to right): Professor Carlo Boldrighini (University of Rome “La Sapienza”), Professor Yuri Kondratiev (Bielefeld University, Germany), Mykola Leonenko (Cardiff University, UK)

This is exactly what I want to call my memories of a Yuri, to remind everyone how easily he walked along Tereschenkivska street to Mathematical Institute, how charmingly he smiled and how infectiously he laughed. It seems to me that an open smile was his essence. Yuri was an amazingly open and pure person. Always helped friends. The memory of him will live in our hearts as long as they knock.  And  here in Cardiff, looking at starry sky, sometimes it seems to me that there is one less star. And sometimes it seems that on the contrary, somewhere near the Milky Way a new little star has appeared.

Dr Dmitri Finkelshtein, Associate Professor, Swansea University, UK

We are young until our teachers are alive. Yuri Kondratiev was luckier than me; he was 66 years old when his teacher, Yuri Berezansky, left this world. And Yuri Kondratiev managed to stay young in spirit all his life. He was indeed a “larger than life” character, as our common friend (and co-author) said after hearing the tragic news.

I became a part of “Kondratiev’s group” as a bachelor student 24 years ago, more than half of my life ago. These years were obviously full of events significant to me, my family, and my country. However, throughout all these years, there was the pivot, Yuri, around whom the research life revolved. Always full of ideas and plans, always enthusiastic, always trying to find unexpected features of, at first glance, well-known objects. The long blackboard in his office in Bielefeld became covered with chalk, and then cleaned, and then covered again, and then we agreed to continue tomorrow, on Saturday – “a good day, there it will be quiet at the Uni” – and then on Sunday again.

He loved his office in Bielefeld and his homes both in Bielefeld and Kyiv. He said that it would be great to take a lift from his Bielefeld office floor to exit at the ground floor of his building in Kyiv, opposite the National Opera. I doubt somebody counted how many research visitors discussed Maths with him in his office during a year, but I do know about a guy who declined to come to Bielefeld once again: “The research life is too intensive here,” he said. There is a popular type of scientific program, “research in peace” – it was not the case; rather it was “research in drive.” But we loved this, and he definitely enjoyed this.

Yuri had a wide scope of interests and knowledge beyond Maths; Yuri had a lot of friends across the world; Yuri opened the scientific doors for many PhD students; many of them have their own ones, “Kondratiev’s scientific grandchildren” – a big research family.

The last years were difficult for him, with retirement, health issues, Covid-time, the war – all these culminated in the tragic year of 2023. Despite this, he worked a lot, finished a lot, and started a lot.

His name will remain in Maths. Not only because of the functional spaces named after him but also because of the directions he initiated, the ideas he conceived, and the books and articles he wrote.

Our Teacher has left us, and though we may no longer be young, I feel rejuvenated every time I remember him.