\Bibitem{Mit49} \by A.~K.~Mitropol'skii \paper Ordinary correlation equations \jour Russian Math. Surveys \yr 1949 \vol 4 \issue 5 \mathnet{http://mi.mathnet.ru//eng/rm8658} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=32146} \zmath{https://zbmath.org/?q=an:0035.08701}