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The Set containing no elements, denoted 
. Strangely, the empty set is both Open and
Closed for any Set 
and Topology. A Groupoid, Semigroup, Quasigroup,
Ringoid, and Semiring can be empty. A Monoid, Group, and Rings must have at least one
element, while Division Rings and Fields must have at least two elements.
References
Conway, J. H. and Guy, R. K. The Book of Numbers. New York: Springer-Verlag, p. 266, 1996.