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# cyclic ring

A ring is a cyclic ring if its additive group is cyclic.

Every cyclic ring is commutative under multiplication. For if $R$ is a cyclic ring, $r$ is a generator of the additive group of $R$, and $s,t\in R$, then there exist $a,b\in{\mathbb{Z}}$ such that $s=ar$ and $t=br$. As a result, $st=(ar)(br)=(ab)r^{2}=(ba)r^{2}=(br)(ar)=ts.$ (Note the disguised use of the distributive property.)

A result of the fundamental theorem of finite abelian groups is that every ring with squarefree order is a cyclic ring.

If $n$ is a positive integer, then, up to isomorphism, there are exactly $\tau(n)$ cyclic rings of order $n$, where $\tau$ refers to the tau function. Also, if a cyclic ring has order $n$, then it has exactly $\tau(n)$ subrings. This result mainly follows from Lagrange’s theorem and its converse. Note that the converse of Lagrange’s theorem does not hold in general, but it does hold for finite cyclic groups.

Every subring of a cyclic ring is a cyclic ring. Moreover, every subring of a cyclic ring is an ideal.

$R$ is a finite cyclic ring of order $n$ if and only if there exists a positive divisor $k$ of $n$ such that $R$ is isomorphic to $k{\mathbb{Z}}_{{kn}}$. $R$ is an infinite cyclic ring that has no zero divisors if and only if there exists a positive integer $k$ such that $R$ is isomorphic to $k{\mathbb{Z}}$. (See behavior and its attachments for details.) Finally, $R$ is an infinite cyclic ring that has zero divisors if and only if it is isomorphic to the following subset of ${\mathbf{M}}_{{2\operatorname{x}2}}({\mathbb{Z}})$:

$\left\{\left.\left(\begin{array}[]{cc}c&-c\\ c&-c\end{array}\right)\right|c\in{\mathbb{Z}}\right\}$

Thus, any infinite cyclic ring that has zero divisors is a zero ring.

# References

- 1
Buck, Warren.
*Cyclic Rings*. Charleston, IL: Eastern Illinois University, 2004. - 2
Kruse, Robert L. and Price, David T.
*Nilpotent Rings*. New York: Gordon and Breach, 1969. - 3
Maurer, I. Gy. and Vincze, J. “Despre Inele Ciclice.”
*Studia Universitatis Babeş-Bolyai. Series Mathematica-Physica*, vol. 9 #1. Cluj, Romania: Universitatea Babeş-Bolyai, 1964, pp. 25-27. - 4
Peinado, Rolando E. “On Finite Rings.”
*Mathematics Magazine*, vol. 40 #2. Buffalo: The Mathematical Association of America, 1967, pp. 83-85.

## Mathematics Subject Classification

13A99*no label found*16U99

*no label found*

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## Comments

## article links

Could somebody help me with this? There are at least two articles by the name of "cyclic", and, in my article, cyclic rings, I would like the word "cyclic" to link with the article on cyclic groups; however, it links to the article on cyclic quadrilaterals instead. Does someone know how to rememdy this???

Wkbj79

## Re: article links

You can use \PMlinkname{cyclic}{CyclicGroup} to get the correct link. (See http://aux.planetmath.org/doc/userlinking.html for information about other ways to control linking.)

## very little errata

It's "Despre Inele Ciclice" and not "Despre Inele Ciclece".

## Re: very little errata

Got it. Thanks.