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Background

Let $R$ be an integral domain and let $S$ be this set of pairs:

$$S=\{(a,b)\mid a,b\in R, b\neq 0\}$$

Define a relation $\sim$ on the set $S$ by

$$(a,b)\sim (c,d)\text{ means }ad-bc \text{ in }R$$.

Theorem 10.30 Let $R$ be an integral domain. Then there exists a field $F$ whose elements are of the form $a/b$ with $a,b\in R$ and $b\neq 0_R$, subject to the equality condition

$$\frac{a}{b}= \frac{c}{d}\text{ in }F\text{ if and only if }ad=bc\text{ in }R$$

Addition and multiplication in $R$ are given by

$$\frac{a}{b} +\frac{c}{d} =\frac{ad+bc}{bd}, \frac{a}{b}\cdot\frac{c}{d} =\frac{ac}{bd},$$

The set of elements in $F$ of the form $a/1_R(a\in R)$ is an integral domain isomorphic to $R$.

....The ring $R$ will be identified with its isomorphic copy in $F$. Then we can say that $R$ is the subset of $F$ consisting of elements of the form $a/1_R$. The field $F$ is called the field of quotients of $R$.

Theorem 6.1 For an ingeral domain $R$, a field $Q(R)$ can be constructed such that:

$1)$ A field $Q(R)$ contains a subring $P$ which is isomorphic to the ring $R$.

$2)$ Each non-zero element of the field $Q(R)$ can be written in the form $ab^{-1}$ where $a,b\in P, b\neq 0$.

Definition 6.2
The field $Q(R)$ constructed in theorem 6.1 is called the field of fractions of a ring $R$.

Definition for multiplicative closed set. Let $R$ be a ring. A subset $T$ of $R$ is a multiplicative system if $1 \in T$, and if $a, b \in T$ implies that $ab \in T$ — that is, T is multiplicatively closed and contains $1.$

If $T$ is a multiplicative system of a ring $R$, then an equivalence relation can be defined on an appropriate set $S$ of "fractions" using elements of $R$,

$$S=\{a/b\mid a,b\in R, b\in T\}$$

by

$$\frac{a}{b}\cong \frac{c}{d}\text{ if and only if }t(ad-bc)=0$$.

For some $t\in T$.

Definition 6.7 Let $R$ be a ring, and let $T$ be a multiplicative system of $R$. The above ring $R_T$ of equivalence classes of fractions from $R$ whose denominators are elements of $T$, with addition and multiplication defined as above, is called the localization of $R$ with respect to $T$, and which we read as "$R$ localized at $T$."

Theorem 10.6.2. Any integral domain can be embedded in a field. (In the proof, $R$ is used to denoted integral domain, $F$ is used for field.)

Definition 10.6.2. For any integral domain $R$, the field $F$ constructed above is called the field of quotients of $R$.

Exercise 15. Let $R$ be a commutative ring with unity. A subset $S$ of $R$ is said to be multiplicative if $1\in S, 0\not\in S$ and $ab\in S$. Define a binary relation $\theta$ on $R\times S$ by

$$(a,s)\theta (b,t) \text{ iff there exists }u\in S\text{ such that }u(at-bc)=0.$$

Prove that $\theta$ is an equivalence relation on $R\times S$. Let the equivalence containing $(a,s)$ be denoted by $a/s$ and let

$$S^{-1}R=\{\frac{a}{s},a\in R, s\in S\}$$

Define addition and multiplication on $S^{-1}R$ by

$$\frac{a}{s} +\frac{c}{t} =\frac{at+bs}{st}, \text{ and } \frac{a}{s}\cdot\frac{c}{t} =\frac{ac}{st}$$

Then prove that $(S^{-1}R,+,\cdot)$ is a commutative ring with unity. This ring is called the ring of fractions of $R$ by $S$.

Question

For the question to follow, i wrote down definitions and theorems from various sources as references in the above backgroubd section. Suppose I have a single variable polynomial ring $F[x]$, a subset $S$ of $F[x]$ and I have a prime ideal $P=(f(x))$, where $f(x)$ is some specific irreducible polynomial with non-zero constant term over $F$.

If I want to localize $F[x]$ at the prime ideal $P$, the correct way to describe the mulitplicative closed set/system $S$ and the ring of fractions $F[x]_P$ in set builder notation, or since $f(x)$ is some specific polynomial, the best that can be done is the following:

Multiplicative closed set: $$S=\{q(x)\mid q(x)\in F[x], q(x)\not\in P=(f(x))\}$$

and the ring of fractions:

$$F[x]_{P=(f(x))}=S^{-1}F[x]=(F[x]\setminus P)^{-1}F[x]= \{[\frac{p(x)}{q(x)}]\mid p(x),q(x)\in F[x], q(x)\in S\},\text{ and where } S=F[x]\setminus P$$.

Is the way I wrote out $S$ and $F[x]_{P=(f(x))}$ correct?

Thank you in advance

Seth
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  • I don't quite understand what you have written. Firstly, it seems to me that the way you have written it, $F[x]_P\subseteq F[x]$? That is not true. Localisation is (usually) defined as a set of equivalence classes of $A\times S$. – ultralegend5385 Feb 12 '25 at 17:44
  • @ultralegend5385 when you say localisation is defined as a set of equivalence classes, does that include the multiplicative closed set? I thought writing out the localisation at a prime involves equivalence classes, but for mulitplicative closed set consist the set of elements that can are invertible and those elements are what is needed for defining the equivalence classes? Can you show me the necessary corrections please. I think from what you are saying, I am a bit confused now over the notations. Also, I did not meant to imply $F[x]_P\subset F[x]$ in my notation. – Seth Feb 12 '25 at 17:51
  • @ultralegend5385 I edit my post. Not sure if it is correct. I should mention that the only other time I saw a concrete example of localization is in Stephen Lovett's abstract algebra text. He was localizating a quotient ring generated by some specific ideal. – Seth Feb 12 '25 at 17:55
  • Have a look at this answer. I suppose it will help you understand localisation better. If you have some specific questions, I can answer those. – ultralegend5385 Feb 12 '25 at 19:04
  • @ultralegen5385 I have to step out, but I will definitely come back tonight and ask you some questions. Sorry about that. I will also edit my post and linked to some other previous posts, it would give you a background of where I am having confusions. thank you in advance for taking the time to answer my questions. – Seth Feb 12 '25 at 19:09
  • @ultralegend5385 Sorry for my late replies. I came home late last night due to giant snow storm. I added definition and theorems from various sources for the topic of field of quotients and also for localization and ring of fractions in the topic of localization. Originally I made a post on how to do calculation in localization of quotient rings here. From the cited example in that post, I was not making headway, then from an exercise of the same source.... – Seth Feb 13 '25 at 17:48
  • @ultralegend5385 ....I asked about what the correct multiplicative closed set should be here. I think the notation $^{−1}$ means I am including elements from the ring $$ and also elements from $$ which is in the multiplicative closed set $$ and after inverting,, they become elements in $^{−1}$. I also added what I think the set $$ should be in my post. Can you check if my edit post is correct please. Thank you in advance. – Seth Feb 13 '25 at 18:08

1 Answers1

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Localisation at a prime ideal $P$ means localising with the multiplicative set $S=R\setminus P$. From the definitions that you have written, you get $$F[x]_P=\left\{\frac{p(x)}{q(x)}\mid q(x)\notin P\right\}$$ I'm not sure how you have gotten $g(x)q(x)+1$, or the condition $\gcd(p(0),p)=1$.

Hope this helps. :)

  • sorry for late reply. I was thinking about this post and I got things mixed up. I edited my post. Can you check if it is correct please. I added what the multiplicately closed set should be also. Thank you in advance. – Seth Feb 17 '25 at 14:34
  • @Seth: Yeah, it's okay now. – ultralegend5385 Feb 17 '25 at 15:00
  • can I also ask you if you can check the correctness of the notation in this post please if you have a moment. I want to try proving the theorem using the standard creating surjective homomorphism and kernel technique. I first have to make sure I am getting the notations correct. Thank you in advance. – Seth Feb 17 '25 at 15:13