Alexej Chervonenkis

A few days ago, Alexej Chervonenkis went for a walk in the forest "Losinoostrovsky" and got lost. He called his relatives in the evening, and last talked to his wife at midnight, saying that would try to sleep a bit. A group of volunteers was organized to start the search. When his body was eventually found, it was too late.

I always felt lucky to work in a field many of whose founders are still alive. Alexej was such a person, a giant in the field of machine learning, whose work with Vladimir Vapnik developed a theory of learning at a time when nobody could imagine that such a theory was possible, let alone how it would look.

Whilst I did not have the priviledge to know Alexej well, I want to share some impressions and pictures, especially for those who could not meet him. Others will comment on his research in detail, and we can all look forward to reading this - I understand that Vladimir will write something for the Russian journal where they published many of their seminal papers.

Vladimir, who was my PhD advisor, spoke about Alexej with great admiration and affection. I believe I first saw Alexej in 1998 at Holloway College, at a memorable meeting that also included Solomonoff, Rissanen, Wallace, and Vapnik. He gave a talk about SVMs where he also mentioned the 'generalized portrait' algorithm, which is in todays nomenclature a linear hard margin SVM for unlabelled data. This inspired my coworkers and me to start working on the one-class SVM. When combined with kernels and the '\nu-trick', it led to a method for estimating quantiles of high-dimensional distributions. I showed this to Vladimir, who in turn made me show it to Alexey, maybe during a Dagstuhl meeting. Alexey patiently listened to my presentation, watched a matlab demo, and at the end he said politely "thank you" and left.

The other lasting memories are from a time much later, autumn 2013. We attended a conference in Moscow. I did not know if he remembered me at all, and he was usually alone during the coffee breaks, maybe occasionally standing with some of his compatriots. When I gave my talk, I saw him in the audience and I have to admit I was slightly concerned what he would think about it - it was about a completely different topic, in causal inference, and the results were nowhere near as beautiful as in the field that had co-founded. Afterwards he approached me, shook my hand, and said "Good morning. Thank you." I emailed Vladimir about this, who replied "I am glad that you have a good time in Moscow and became friend with Alexey."

During the same meeting, there was an organized tour of Kolmogorov's datcha. A bus had taken us there - it was fascinating - and when we got on the bus to go back, Alexej apparently indicated that he was going to walk. Someone tried to convince him to join the bus (it was quite a distance), but he would not even argue and instead just started walking. It was raining a little, and the bus overtook him as he walked along the road, perfectly at ease with himself. We saw him again a few hours later at dinner - we had meanwhile visited the Kremlin. Vladimir conveyed the impression to me that for Alexej, walking had a spiritual quality, and he had an uncanny sense of orientation. During his life, he walked all streets of Moscow, and he had begun trying to walk all its forests, too.

Just a few days later we met again, at a meeting in Cyprus to celebrate Alexej's 75th birthday with symposium in his honor. After we had given the talks, the afternoon was free and I wanted to go for a swim. On the way down, I saw Alexej. I figured he also wanted to swim and I told him which beach the hotel had recommended to me (a short walk from the hotel, with a warning that it's not safe). Alexej was unmoved by this and instead started heading for a place that too me looked absurd. I decided that I should better join him. We ended up going into the water in a place where I definitely wouldn't have gone swimming (and I consider myself a good swimmer). Already on the way in, the first waves threw him over and I helped him up, but that would not stop him. Once we were in the water, I noticed a strong current and we quickly were pulled away from the tiny beach towards the part of the shore that was rocky. Alexej had an unusual swimming style, lying sideways in the water and swimming with only one arm. I got seriously worried how we should get out of this. In the end, he decided to just swim towards the rocks, until the waves washed us upon the rocks. We had to hold on to each other, and emerged with a few scratches. Amazingly, he did not show any fear whatsoever. When it was time to say bye-bye the next day, he was silent as usual, but he gave me a knowing smile that to me expressed an unspoken understanding of what we had experienced. This is the last time I saw him, and how I will remember him. I am grateful to have met him.

B. Schölkopf

Alexej Chervonenkis

Gammerman, Schölkopf, Dudley, Chervonenkis