1. Birkhof, G., MacLane, S.: A Survey of Modern Algebra. MacMillan Publishers, London (1970)
2. Hopcroft, J., Ullman, J.: Introduction to Automata Theory, Languages and Computation (1979)
3. Kearfott, R.B.: Interval Computations: Introduction, Uses, and Resources.
https://interval.louisiana.edu/preprints/survey.pdf
4. Kelley, J.: General Topology, Van Nostrand (Chap. 3, Theorem 1, Item e) (1955)
5. Kuroki, N.: On power semigroups. Proc. Japan Acad. 47, 449 (1971)