In mathematics, a trivial group or zero group is a group consisting of a single element. All such groups are isomorphic, so one often speaks of the trivial group. The single element of the trivial group is the identity element and so it is usually denoted as such: 0, 1 or e depending on the context. If the group operation is denoted ∗ then it is defined by e ∗ e = e.
The similarly defined trivial monoid is also a group since its only element is its own inverse, and is hence the same as the trivial group.
The trivial group should not be confused with the empty set (which has no elements, and lacking an identity element, cannot be a group).
Given any group G, the group consisting of only the identity element is a subgroup of G, and, being the trivial group, is called the trivial subgroup of G.
The trivial group is cyclic of order 1; as such it may be denoted Z1 or C1. If the group operation is called addition, the trivial group is usually denoted by 0. If the group operation is called multiplication then 1 can be a notation for the trivial group. (Combining these leads to the trivial ring in which the addition and multiplication operations are identical and 0=1.)