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- 1 Definition
- 2 Arithmetic functions
- 3 Group properties
- 4 Endomorphisms
- 5 Subgroups
- 6 Quotients
- 7 Other constructions
- 8 In larger groups
- 9 GAP implementation
- 10 Internal links
The cyclic group of order 3 is defined as the unique group of order 3. Equivalently it can be described as a group with three elements where with the exponent reduced mod 3. It can also be viewed as:
- The quotient group of the group of integers by the subgroup of multiples of 3
- The multiplicative group comprising the three cuberoots of unity (as a subgroup of the multiplicative group of nonzero complex numbers)
- The alternating group on three elements
- The group of orientation-preserving symmetries (rotational symmetries) of the equilateral triangle
- The multiplicative group of the field of four elements. In particular, it is the general linear group .
Want to compare and contrast arithmetic function values with other groups of the same order? Check out groups of order 3#Arithmetic functions
|Group of prime order||Yes||By definition|
|Cyclic group||Yes||By definition||Smallest odd-order cyclic group|
|Elementary abelian group||Yes||By definition|
|Abelian group||Yes||Cyclic implies abelian|
|Nilpotent group||Yes||Abelian implies nilpotent|
|Metacyclic group||Yes||Cyclic implies metacyclic|
|Supersolvable group||Yes||Cyclic implies supersolvable|
|Solvable group||Yes||Abelian implies solvable|
|Simple group||Yes||Simple abelian if and only if prime order|
Any endomorphism of a cyclic group is determined by where it sends the generator. The cyclic group of order three has three endomorphisms:
- The identity map is an endomorphism. This map sends every element to itself.
- The square map is an endomorphism. This map sends to , to , and to itself.
- The trivial map is an endomorphism. This map sends every element to .
Of the three endomorphisms, two are automorphisms: the identity map and the square map. These form a cyclic group of order two: the square map, applied twice, gives the identity map.
The group has no proper nontrivial subgroups: the only subgroups are the whole group and the trivial subgroup.
More generally, any nontrivial group with no proper nontrivial subgroup must be cyclic of prime order. Conversely, any cyclic group of prime order has no proper nontrivial subgroup. Further information: No proper nontrivial subgroup implies cyclic of prime order
The cyclic group of order three has only two quotients: the whole group and the trivial quotient. This follows from the fact that this group is simple -- it has no proper nontrivial normal subgroup.
The holomorph of this group, i.e., the semidirect product of this group with its automorphism group, is isomorphic to the symmetric group on three letters. The cyclic group sits inside this as the alternating group and the automorphism group sits inside as a subgroup of order two.
In larger groups
Occurrence as a subgroup
The cyclic group of order 3 occurs as a subgroup in many groups. In general, a group contains a cyclic subgroup of order three if and only if its order is a multiple of three (this follows from Cauchy's theorem, a corollary of Sylow's theorem).
Occurrence as a normal subgroup
The cyclic group of order three occurs as a normal subgroup in some groups.
For instance, if a field contains non-identity cuberoots of unity, then the multiplicative group of the field contains a cyclic subgroup of order three. As a corollary, the general linear group contains a central subgroup of order three.
Normal subgroups of order three need not be central; for instance, in the symmetric group on three letters, the alternating group is a normal subgroup of order three, but is not central. However, for an odd-order group, any normal subgroup of order three is central. This follows from a more general fact: a normal subgroup whose order is the least prime divisor of the order of the group is central.
This finite group has order 3 and has ID 1 among the groups of order 3 in GAP's SmallGroup library. For context, there are groups of order 3. It can thus be defined using GAP's SmallGroup function as:
For instance, we can use the following assignment in GAP to create the group and name it :
gap> G := SmallGroup(3,1);
Conversely, to check whether a given group is in fact the group we want, we can use GAP's IdGroup function:
IdGroup(G) = [3,1]
or just do:
to have GAP output the group ID, that we can then compare to what we want.
The cyclic group of order three can be defined using the CyclicGroup function: