\Bibitem{Kat67} \by A.~B.~Katok \paper Spectral properties of dynamical systems with an integral invariant on the torus \jour Funktsional. Anal. i Prilozhen. \yr 1967 \vol 1 \issue 4 \pages 46--56 \mathnet{http://mi.mathnet.ru/faa2843} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=237903} \zmath{https://zbmath.org/?q=an:0172.12102} \transl \jour Funct. Anal. Appl. \yr 1967 \vol 1 \issue 4 \pages 296--305 \crossref{https://doi.org/10.1007/BF01076009}