Leon Ehrenpreis, recollections from the recent decades

Leon Ehrenpreis was an outstanding world class mathematician and a wonderful, warm person. I had a privilege to consider myself his friend for the last two decades. It is hard to do justice to his manifold mathematics and personality, but I will try to at least add some recollections to this tribute volume. Leon Ehrenpreis has been one of my mathematical heroes for about 40 years. I first encountered his, Lars Hörmander’s, Bernard Malgrange’s and Victor Palamodov’s fundamental and beautiful works on systems of linear constant coefficient PDEs in early 1970s, when I was an undergraduate student and then a PhD candidate. They have had a profound impact on me, in particular when working on the Floquet theory of periodic PDEs, which we with Leonid Zelenko started developing in a few years. I am sure that Bernard Malgrange and Daniele Struppa have described this part of Leon’s legacy much better than I ever could. I will only address some of the research Leon pursued in the last two decades of his life, which I was lucky to witness. Some time around 1988, a medical industry contract forced me to learn the basics of a fascinating topic that I had never heard of before, the so called computed tomography. This turned out to be fateful. Our research group in Voronezh found the mathematics of tomography so challenging and exciting, that in the following decades several of us have being devoting a significant part of time working on tomographic problems. Appearance in the 80s of the Russian translation of the cornerstone book on this topic by Frank Natterer [52] also helped. Interestingly enough, I discovered that several mathematicians whom I admired for their work in completely different