An open interval is an interval that does not include its end points. The open interval is denoted , although the nonstandard notation is sometimes also used.

Croft, H. T.; Falconer, K. J.; and Guy, R. K. Unsolved Problems in Geometry. New York: Springer-Verlag, p. 1, 1991.

Gemignani, M. C. Elementary Topology. New York: Dover, 1990.

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Weisstein, Eric W. "Open Interval." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/OpenInterval.html