\Bibitem{Vil52} \by N.~Ya.~Vilenkin \paper On the theory of orthogonal kernels \jour Russian Math. Surveys \yr 1952 \vol 7 \issue 3 \mathnet{http://mi.mathnet.ru//eng/rm8322} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=52551} \zmath{https://zbmath.org/?q=an:0048.08301}