\Bibitem{Vas01} \by A.~B.~Vasil'eva \paper On some periodic contrast structures \jour Mat. Model. \yr 2001 \vol 13 \issue 12 \pages 43--45 \mathnet{http://mi.mathnet.ru/mm815} \mathscinet{https://mathscinet.ams.org/mathscinet-getitem?mr=1904031} \zmath{https://zbmath.org/?q=an:1017.35015}