↑ Dixmier, Jacques (1984). General Topology. Undergraduate Texts in Mathematics. Translated by Sterling K. Berberian. Springer. tr. 6. ISBN0-387-90972-9. According to this definition, an open neighborhood of x is nothing more than an open subset of E that contains x.
Kelley, John L. (1975). General topology. New York: Springer-Verlag. ISBN0-387-90125-6.
