Number of conjugacy classes

This article defines an arithmetic function on groups
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Definition

The number of conjugacy classes in a group is defined as the number of conjugacy classes, viz the number of equivalence classes under the equivalence relation of being conjugate.

Related group properties

A group in which every conjugacy class is finite is termed an FC-group. In particular, a FC-group is finite if and only if it has finitely many conjugacy classes.
A group with two conjugacy classes is a nontrivial group with exactly one conjugacy class of non-identity elements. Note that the only such finite group is cyclic group:Z2. However, there are many infinite groups with this property.

Facts

Ways of measuring this number for a finite group

The number of conjugacy classes in a finite group equals the number of equivalence classes of irreducible representations. Further information: number of irreducible representations equals number of conjugacy classes
The number of conjugacy classes is the product of the order of the group and the commuting fraction of the group, which is the probability that two elements commute. This follows from the orbit-counting theorem. Further information: commuting fraction, equivalence of definitions of commuting fraction

Lower bounds on the number of conjugacy classes

The number of conjugacy classes in a group is at least as much as the number of elements in the center.
The number of conjugacy classes in a group is at least as much as the number of elements in the abelianization.
There are finitely many finite groups with bounded number of conjugacy classes. Thus, the number of conjugacy classes puts an upper bound on the order of a finite group. Conversely, the order of a group puts a lower bound on the number of conjugacy classes. On the other hand, there do exist infinite groups with only finitely many conjugacy classes.

Upper bounds on the number of conjugacy classes

The number of conjugacy classes is at most as much as the number of elements in the group. Equality (for finite groups) holds if and only if the group is abelian.
Commuting fraction more than five-eighths implies abelian: In particular, this means that for a finite non-abelian group, the number of conjugacy classes is bounded by $5 / 8$ times the order of the group.

Relating the number of conjugacy classes for a group with its subgroups and quotients

Number of conjugacy classes in a subgroup may be more than in the whole group
Number of conjugacy classes in a quotient is less than or equal to number of conjugacy classes of group
Commuting fraction in subgroup is at least as much as in whole group: This in particular implies that the quotient of the number of conjugacy classes to the order is at least as much in a subgroup as in the whole group. Thus, the number of conjugacy classes in a subgroup puts an upper bound (via multiplication by the index of the subgroup) on the number of conjugacy classes in the whole group, and the number of conjugacy classes in the whole group puts a lower bound (via division by the index of the subgroup) on the number of conjugacy classes in the subgroup.
Commuting fraction in quotient group is at least as much as in whole group: This in particular implies that the quotient of the number of conjugacy classes to the order is at least as much in a quotient group as in the whole group.

Particular cases

By number of conjugacy classes

Further information: There are finitely many finite groups with bounded number of conjugacy classes

Number of conjugacy classes	List of all finite groups with that number	List of orders of these groups
1	trivial group	1
2	cyclic group:Z2	2
3	cyclic group:Z3, symmetric group:S3	3, 6
4	cyclic group:Z4, Klein four-group, dihedral group:D10, alternating group:A4, more?	4, 4, 10, 12
5	cyclic group:Z5, dihedral group:D8, quaternion group, dihedral group:D14, general affine group:GA(1,5), SmallGroup(21,1), symmetric group:S4, alternating group:A5	5, 8, 8, 14, 20, 21, 24, 60

By groups

Group	Order	Second part of GAP ID	Number of conjugacy classes	Comment
trivial group	1	1	1
cyclic group:Z2	2	1	2	for an abelian group, number of conjugacy classes equals number of elements
cyclic group:Z3	3	1	3	for an abelian group, number of conjugacy classes equals number of elements
cyclic group:Z4	4	1	4	for an abelian group, number of conjugacy classes equals number of elements
cyclic group:Z5	5	1	5	for an abelian group, number of conjugacy classes equals number of elements
symmetric group:S3	6	1	3	cycle type determines conjugacy class, so number of conjugacy classes equals number of unordered integer partitions for a symmetric group. See also element structure of symmetric group:S3 and element structure of symmetric groups
cyclic group:Z6	6	2	6	for an abelian group, number of conjugacy classes equals number of elements
cyclic group:Z7	7	1	7	for an abelian group, number of conjugacy classes equals number of elements
cyclic group:Z8	8	1	8	for an abelian group, number of conjugacy classes equals number of elements
direct product of Z4 and Z2	8	2	8	for an abelian group, number of conjugacy classes equals number of elements
dihedral group:D8	8	3	5	See element structure of dihedral groups (for general formula: $(n + 6) / 2$ for dihedral group of order $2 n$ for even $n$ ) and element structure of dihedral group:D8 for specific information. See also nilpotency class and order determine conjugacy class size statistics for groups up to prime-fourth order
quaternion group	8	4	5	See element structure of dicyclic groups (for general formula) and element structure of quaternion group for specific information. See also nilpotency class and order determine conjugacy class size statistics for groups up to prime-fourth order
elementary abelian group:E8	8	5	8	for an abelian group, number of conjugacy classes equals number of elements
cyclic group:Z9	9	1	9	for an abelian group, number of conjugacy classes equals number of elements
elementary abelian group:E9	9	2	9	for an abelian group, number of conjugacy classes equals number of elements
dihedral group:D10	10	1	4	See element structure of dihedral groups (for general formula: $(n + 3) / 2$ for dihedral group of order $2 n$ for odd $n$ )
cyclic group:Z10	10	2	10	for an abelian group, number of conjugacy classes equals number of elements
cyclic group:Z11	11	1	11	for an abelian group, number of conjugacy classes equals number of elements

By family

Family name	Parameter value	Number of conjugacy classes	More information
symmetric group $S_{n}$	$n$	number of unordered integer partitions $p (n)$	element structure of symmetric groups, cycle type determines conjugacy class
dihedral group $D_{2 n}$	$n$ (degree, half the order)	$(n + 3) / 2$ for $n$ odd, $(n + 6) / 2$ for $n$ even	element structure of dihedral groups
alternating group $A_{n}$	$n$	$2 A + B$ where $A$ is the number of self-conjugate integer partitions of $n$ and $B = (p (n) - A) / 2$ is the number of conjugate pairs of non-self-conjugate unordered integer partitions	element structure of alternating groups
general linear group of degree two $G L (2, q)$ over a finite field	$q$ (size of field)	$q^{2} - 1$	element structure of general linear group of degree two
projective general linear group of degree two $P G L (2, q)$ over a finite field	$q$ (size of field)	$q + 2$ if $q$ odd, $q + 1$ if $q$ a power of $2$	element structure of projective general linear group of degree two
special linear group of degree two $S L (2, q)$ over a finite field	$q$ (size of field)	$q + 4$ if $q$ odd, $q + 1$ if $q$ a power of $2$	element structure of special linear group of degree two
projective special linear group of degree two $P S L (2, q)$ over a finite field	$q$ (size of field)	$(q + 5) / 2$ if $q$ odd, $q + 1$ if $q$ a power of $2$	element structure of projective special linear group of degree two