A cyclic group is a special type of group generated by a single element. If the generator of a cyclic group is given, then one can write down the whole group. Cyclic groups are also known as monogenous groups. In this article, we will learn about cyclic groups.

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## Definition of Cyclic Groups

A group (G, $\circ$) is called a cyclic group if there exists an element a∈G such that G is generated by a. In other words,

G = {a^{n} : n ∈ Z}.

The element a is called the generator of G. Mathematically, it is written as follows: G=<a>. If G is an additive cyclic group that is generated by a, then we have G = {na : n ∈ Z}. The following are a few examples of cyclic groups.

- (Z, +) is a cyclic group. Its generators are 1 and -1.
- (Z
_{4}, +) is a cyclic group generated by $\bar{1}$. It is also generated by $\bar{3}$.

Non-example of cyclic groups: Klein’s 4-group is a group of order 4. It is not a cyclic group.

## Order of a Cyclic Group

Let (G, $\circ$) be a cyclic group generated by a. The order of group G is equal to the order of the element a in G. In other words, $|G|=|a|$, where $|g|$ denotes the order of the element g. Depending upon whether the group G is finite or infinite, we say G to be a finite cyclic group or an infinite cyclic group.

In the above example, (Z_{4}, +) is a finite cyclic group of order 4, and the group (Z, +) is an infinite cyclic group.

## Properties of Cyclic Groups

- If a cyclic group is generated by a, then it is also generated by a
^{-1}. - Every cyclic group is abelian (commutative).
- If a cyclic group is generated by a, then both the orders of G and a are the same.
- Let G be a finite group of order n. If G is cyclic then there exists an element b in G such that the order of b is n.
- Let G be a finite cyclic group of order n and G=<a>. Then G=<a
^{r}> if and only if r<n and gcd(r, n)=1. Thus the number of generators of a finite cyclic group of order n is Φ(n), where Φ is the Euler-Phi function. - Every subgroup of a cyclic group is also cyclic.
- A cyclic group of prime order has no proper non-trivial subgroup.
- Let G be a cyclic group of order n. Then G has one and only one subgroup of order d for every positive divisor d of n.
- If an infinite cyclic group G is generated by a, then a and a
^{-1}are the only generators of G.

## Problems and Solutions on Cyclic Groups

**Question 1:** Find all subgroups of the group (Z, +).

**Answer:**

We know that (Z, +) is a cyclic group generated by 1. As every subgroup of a cyclic group is also cyclic, we deduce that every subgroup of (Z, +) is cyclic, and they will be generated by different elements of Z.

The cyclic subgroup generated by the integer m is (mZ, +), where mZ={mn: n ∈ Z}. As (mZ,+) = (-mZ, +) we conclude that all the subgroups of the group (Z, +) are given as follows:

{(mZ, +) : m is a non-negative integer}.

**Question 2:** Prove that (Q, +) is not a cyclic group.

**Answer:**

For a contradiction, we assume that (Q, +) is a cyclic group generated by a. Since (Q, +) is an additive group, we can write it as Q={na : n∈ Z}. As $\frac{1}{2}$a ∈ Q and see that $\frac{1}{2}$a does not belong to the set Q={na : n∈ Z}. Thus a cannot a generator of (Q, +). As a result, the additive group Q is non-cyclic.

**Question 3:** Prove that (R, +) is not a cyclic group.

**Answer:**

If possible, we assume that (R, +) is a cyclic group. Note that (Q, +) is a subgroup of (R, +). Thus being a subgroup of a cyclic group, we obtain that (Q, +) is a cyclic group. Thus we arrive at a contradiction because (Q, +) is non-cyclic follows from the above question. So the additive group of real numbers is not a cyclic group.

Question | Answer |

Is Z cyclic? | As any integer can be written as n= 1+1+…+1 (n times), we see that (Z, +) is generated by 1. Hence, Z is a cyclic group. |

Give an example of a non-cyclic group of order 4. | The group Z_{2} × Z_{2} has order 4 and it has no element of order 4. So Z_{2} × Z_{2} is a noncyclic group of order 4. |

Give an example of a cyclic group of order 4. | Z_{4} is a cyclic group of order 4 generated by $\overline{1}$. |

**Related Topics:**

**Group Theory: Definition, Examples, Orders, Types, Properties, Applications**

**Abelian Group: Definition, Properties, Examples**

## FAQs on Cyclic Groups

**Q1: Is the symmetric group S**

_{3}cyclic?Answer: Since S_{3} is not an abelian group, S_{3} is not a cyclic group.

**Q2: Is the dihedral group D**

_{4}cyclic?Answer: We know that the dihedral group is non-abelian. Thus D_{4} is not a cyclic group.

This article is written by Dr. T. Mandal, Ph.D in Mathematics. On Mathstoon.com you will find Maths from very basic level to advanced level. Thanks for visiting.