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A tribute to the 70th birthday of Professor Victor Burenkov

This volume is dedicated to the 70th birthday of the outstanding mathematician Victor Ivanovich Burenkov. With a broad and diverse range of interests Victor has made major contributions to many areas of both pure and applied mathematics, and has more than 150 publications in leading international journals. He is a leading authority on real analysis and the theory of function spaces, especially on Sobolev spaces and spaces with fractional order of smoothness and their applications. He has also done important work on partial differential equations and integral equations including, in particular, his contributions to the theory of hypoelliptic equations and the spectral theory of partial differential operators, and his work on ill-posed problems. Victor’s early mathematical training was in Moscow, at the Moscow Institute of Physics and Technology and the Steklov Institute of Mathematics of the Russian Academy of Sciences. His research interests were formed under the influence of his teacher, the great Russian mathematician Sergei Mikhailovich Nikol’skii. V.I. Burenkov has developed a number of ingenious approaches and methods that led to significant breakthroughs. His method of mollifiers with variable step and translation allowed him to obtain seminal results in the approximation of functions from general function spaces by various classes of infinitely differentiable functions and especially in the extension