Empty Set
The Set containing no elements, denoted 
. Strangely, the empty set is both Open andClosed for any Set 
and Topology. A Groupoid, Semigroup, Quasigroup,Ringoid, and Semiring can be empty. A Monoid, Group, and Rings must have at least oneelement, 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.
- Cover
- Covering
- Covering Dimension
- Covering System
- Cover Relation
- Coversine
- Coxeter Diagram
- Coxeter-Dynkin Diagram
- Coxeter Graph
- Coxeter Group
- Coxeter Matrix
- Coxeter's Loxodromic Sequence of Tangent Circles
- Coxeter-Todd Lattice
- Cox's Theorem
- Cramer's Rule
- Cramér Conjecture
- Cramér-Euler Paradox
- Cramér's Theorem
- Craps
- CRC
- Creative Telescoping
- Cremona Transformation
- Cribbage
- Criss-Cross Method
- Criterion