Dixmier, Jacques (1984). General Topology. Undergraduate Texts in Mathematics. Translated by Sterling K. Berberian. Springer. hlm. 6. ISBN0-387-90972-9. According to this definition, an open neighborhood of is nothing more than an open subset of that contains
Mashadi dan Hadi, Abdul (2017). Analisis I(PDF). UR Press Pekanbaru. hlm. 60. ISBN978-979-792-796-7.Parameter |url-status= yang tidak diketahui akan diabaikan (bantuan)