1. P.Wigner, Symmetries and reflections. BloomingtonLondon: Indiana University Press, 1970
2. Toroidal Space, http://harmonicresolution.com/Toroidal%20Space.htm
3. Manifold, https://en.wikipedia.org/wiki/Manifold
4. Joseph Rotman, Galois Theory (Second ed.). Springer, 1998, doi:10.1007/978-1-4612-0617-0
5. V. Riznyk, Synthesis of optimum combinatorial systems. Lviv: High School, 165p., 1989 (in Ukrainian)