(Ingelesez)Harper, J.F.. «Defining continuity of real functions of real variables» BSHM Bulletin: Journal of the British Society for the History of Mathematics: 1–16. doi:10.1080/17498430.2015.1116053..
(Ingelesez)Speck, Jared. (2014). Continuity and Discontinuity. , 3 or. Aipua: «Example 5. The function 1/x is continuous on (0, ∞) and on (−∞, 0), i.e., for x > 0 and for x < 0, in other words, at every point in its domain. However, it is not a continuous function since its domain is not an interval. It has a single point of discontinuity, namely x = 0, and it has an infinite discontinuity there.».