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.
Conway, J. H. and Guy, R. K. The Book of Numbers. New York: Springer-Verlag, p. 266, 1996.