# The Nature of Principal Ideals

• Mar 15th 2013, 06:19 AM
Bernhard
The Nature of Principal Ideals
Fraleigh (A First Course in Abstract Algebra) defines principal ideals in section 27 on page 250. His definition is as follows:

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"27.21 Definition

If R is a commutative ring with unity and $a \in R$ , the ideal $\{ ra | r \in R \}$ of all multiples of a is the principal ideal generated by a and is denoted <a>.

An ideal N of R is a principal ideal if N = <a> for some $a \in R$

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Consider $N =\{ ra | r \in R \}$ ...........................(1)

If we take r = a in (1) then we have $ra = aa = a^2 \in N$

If we take r = a and $a^2 \in N$ the we have using (1) again that $ra = a^2 a = a^3 \in N$

Continuing this, then we have $a, a^2, a^3, a^4, a^5$ , .... all belonging to N along with the other elements where $r \ne a$

Is the above analysis correct regarding the nature of principal ideals?

Would really appreciate this issue being clarified.

Peter
• Mar 15th 2013, 05:22 PM
Gusbob
Re: The Nature of Principal Ideals
Yes. An ideal is also closed under addition (in fact, it is a subgroup of $(R,+)$), so you also get elements such as $a^2+ a^7 +ra+sa$ with $r,s\in R$. However, you can see that this is the same as $(a+a^6+r+s)a$, where $(a+a^6+r+s)\in R$. Since multiplication by an element in $R$ and adding elements in $N$ generates the whole ideal, this shows (by example, not proof) that $\langle a \rangle = \{ra|r\in R\}$
• Mar 15th 2013, 05:26 PM
Bernhard
Re: The Nature of Principal Ideals
Thanks Gusbob ... That post was really helpful in characterizing the other elements you mentioned!!!