1. G. S. Osipenko and M. B. Ampilova, Introduction to Symbolic Analysis of dynamical Systems (St.-Peterb. Gos. Univ., St. Petersburg, 2005) [in Russian].
2. A. P. Krishchenko, Dokl. Math. 55, 176–178 (1997) [Dokl. Akad. Nauk 353, 17–19 (1997)].
3. A. P. Krishchenko, Differ. Equations 41, 1669–1676 (2005) [Differ. Uravn. 41, 1597-1604 (2005)].