\Bibitem{AdaBerBog78} \by V.~M.~Adamyan, Yu.~M.~Berezanskii, N.~N.~Bogolyubov, I.~S.~Iokhvidov, A.~N.~Kolmogorov, M.~A.~Lavrent'ev, Yu.~A.~Mitropol'skii \paper Mark Grigor'evich Krein (on his seventieth birthday) \jour Russian Math. Surveys \yr 1978 \vol 33 \issue 3 \pages 185--193 \mathnet{http://mi.mathnet.ru/eng/rm3451} \crossref{https://doi.org/10.1070/RM1978v033n03ABEH002490} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=490843} \zmath{https://zbmath.org/?q=an:0381.01009}