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.

© 1996-9

1999-05-25