The set containing no elements, commonly denoted 
or 
, the former of which is used in this work. These correspond
to Wolfram Language and TeX characters
summarized in the table below.
| symbol | TeX | Wolfram Language |
 | \varnothing | \[Diameter] |
 | \emptyset | \[EmptySet] |
Unfortunately, some authors use the notation 0 instead of 
for the empty set (Mendelson 1997). The empty set is
generally designated using 
(i.e., the empty list) in the Wolfram
Language.
A set that is not the empty set is called a nonempty set. The empty set is sometimes also known as the null set (Mendelson 1997).
The complement of the empty set is the universal set.
Strangely, the empty set is both open and closed for any set 
and topology.
A groupoid, semigroup, quasigroup, ringoid, and semiring can be empty. Monoids, groups, and rings must have at least one element, while division algebras and fields must have at least two elements.
