I Equivalent definitons for primitive polynomials

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##\textbf{Definition 1:}## Let ##R## be an UFD. A nonzero polynomial in ##R[x]## is said to be ##\textbf{primitive}## if the only constants that divides it are the units in ##R##.

##\textbf{Definition 2:}## Let ##R## be an UFD. Hence highest common factors of finite subsets of ##R## exists, by collecting common factors of the unique factorization of the elements into products of irreducibles.A polynomial ##f(x)\in R[x]## is called ##\textbf{primitive}## if the highest common factors of the non-zero coefficients of ##f(x)## is equal to ##1##. This implies that if ##u\in R## and ##u|f(x)## then ##u## is a unit in ##R.##

##\textbf{Exercise:}## Prove that a polynomial is ##\textbf{primitive}## if and only if ##1_R## is a greatest common divisor of its coefficients. This property is often taken as the definition of primitive.

##\textit{Hint:}## Since ##c_1c_2\cdots c_mf(x)=g(x),## each ##c_i## divides ##g(x).## Therefore, ##c_i## is a unit in ##R##, because ##g(x)## is primitive.

For ##\textbf{Exercise}##, it says the statement in the Exercise can be taken to be the definition of primitive polynomial instead of ##\textbf{Definition 1}##. But according to ##\textbf{Definition 2}##, it seems that the statement of ##\textbf{Exercise}## implies ##\textbf{Definition 1}##. Regardless if it is an implication or an equivalent statements. How does one go about proving either?

Another thing is, how does gcd related to units in a commutative ring, ED, PID or UFD?

Thank you in advance.
 
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I am confused too, and my book actually defines the statement of the exercise as the definition of a primitive polynomial.

I have difficulty seeing where this statement is any different from definition 2 since the zero coefficients do not change anything.

So do you want to prove the equivalence of the two definitions?
 
@fresh_42 yes, how would one go about proving the equivalence of the two statements. Also i replied to two of my past thread tagging you that you respond to recently.
 
@fresh_42 the posts are here and here. For the second one, I want to know how to correctly generalized the example in my original post and also, how to count the equivalence classes. I asked grad students and they told me they don't know enough about that specific topic. 😞.
 
elias001 said:
@fresh_42 the posts are here and here. For the second one, I want to know how to correctly generalized the example in my original post and also, how to count the equivalence classes. I asked grad students and they told me they don't know enough about that specific topic. 😞.
You make me dizzy with all these cross references. I'm old. Let us deal with one thread at a time and simple reply in those threads. I will get an automatic notice. No tags necessary.
 
elias001 said:
@fresh_42 yes, how would one go about proving the equivalence of the two statements. Also i replied to two of my past thread tagging you that you respond to recently.
First list what we have:

We have a polynomial ##f(x)=a_0x^n+a_1x^{n-1}+\ldots+a_{n-1}x+a_n.## For simplicity, say ##S=\{a_0,a_1,\ldots,a_n\}## and be ##R^0## the units of ##R.##

primitive (def 1) says: ##f(x)## is primitive iff ##\left[\;g(x) \,|\,f(x) \Longrightarrow g(x)\in R^0\;\right]##

Note that ##g(x)\in R^0## means two different things: first, that ##g(x)\in R## and second, that it can be inverted.

primitive (def 2) says: The greatest common divisor of elements in ##S## is ##1.##

Note that it should better be said that ##\operatorname{gcd}S\in R^0## since we can always have other units as divisors. But we don't care about units, so the author says ##\operatorname{gcd}S=1## because it is easier not to deal with obsolete elements like units.

I leave it to you to prove:
a) ##f(x) ## is primitive (def 1) ##\Longrightarrow## ##f(x) ## is primitive (def 2)
and
b) ##f(x) ## is primitive (def 2) ##\Longrightarrow## ##f(x) ## is primitive (def 1).
 
@fresh_42 i don't get how the gcd of the coefficients of a polynomial in a polynomial ring relates to units in the said polynomial ring. Also for the case of units, it doesn't mean gcd has to be ##1##?
 
elias001 said:
@fresh_42 i don't get how the gcd of the coefficients of a polynomial in a polynomial ring relates to units in the said polynomial ring.
The units in ##R[x]## and the units in ##R## are identical. There are no invertible polynomials except constant polynomials, which are already invertible in ##R.## Adding the indeterminate ##x## doesn't change this unless you do not also add ##1/x## to your ring.
elias001 said:
Also for the case of units, it doesn't mean gcd has to be ##1##?
Forget that unit thing. Just pretend ##1## is the only unit. If we had the polynomial ring ##\mathbb{Q}[x]## over the rational numbers, then any non-zero rational number would be a unit. Strictly speaking, ##\dfrac{1}{7}x## would be primitive according to definition 1 but not according to definition 2. That doesn't make sense. It also doesn't make sense to bother about the unit ##\dfrac{1}{7}## since this makes no significant difference. So definition 2 is only a bit sloppy here if it only allows ##1## as divisor and not ##\dfrac{1}{7}.##

Try to concentrate on the logic and not on units.
 
@fresh_42 let's do ##(a)## first.

We assume that ##f(x)## satisfies def 1 for being a primitive polynomial. Let ##d## be the gcd of the coefficients of ##f(x)##, meaning ##d=\text{gcd}(a_n,a_{n-1},\ldots,a_0).## Let ##S=\{a_n,a_{n-1},\ldots,a_0\},## and each ##a_i\in S## is being divided by ##d##, then ##a_i=dc_i## for some element ##c_i\in R##. Then ##c_i|a_i,## for ##i=0,1,2,\ldots,n.## So ##c_i|f(x)##, and ##f(x)=c_ig(x)## for some ##g(x)\in R[x]##. We have ##dc_i-c_i=0## implying ##c_i(d-1)=0## which gives ##d=1##.

I am not sure how to justify that ##c_i## are units in ##R##?
 
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  • #10
elias001 said:
@fresh_42 let's do ##(a)## first.

We assume that ##f(x)## satisfies def 1 for being a primitive polynomial. Let ##d## be the gcd of the coefficients of ##f(x)##, meaning ##d=\text{gcd}(a_n,a_{n-1},\ldots,a_0).## Let ##S=\{a_n,a_{n-1},\ldots,a_0\},## and each ##a_i\in S## is being divided by ##d##, then ##a_i=dc_i## for some unit ##c_i\in R##. Since ##c_i|f(x)##, then ##f(x)=c_ig(x)## for some ##g(x)\in R[x]##. We have ##dc_i-c_i=0## implying ##c_i(d-1)=0## which gives ##d=1##.

I am not sure how to justify that ##c_i## are units in ##R##?
Me neither. And you do not need it.

We must show definition 2. So we assume that ##\operatorname{gcd}S=d.## But ##d## divides also ##f(x)## and by definition 1, this means that ##d## is a unit in ##R.## But if ##\operatorname{gcd}S=d## then we also have ##\operatorname{gcd}S=d\cdot d^{-1}=1## since units don't change divisibility properties. And that is what definition 2 requires.
 
  • #11
@fresh_42 I just edited my post, does it fix the problem. I am not sure I can just do what you suggest: that since ##d## is a gcd, then it is an unit. I try to just show that ##d=1## according to definition 2. I mean that is the requirement for ##f## to be a primitive polynomial? Also Def 1 states: "...if the only constants that divides it are the units in...". So I think it is require that I need to show that ##d## is an unit, since there are might be other elements that divides ##f(x)## that are not units. In def 1, it doesn't state that gcd are units. Am I reading too much into the definition or do I need to find some way to avoid that issue all together.
 
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  • #12
elias001 said:
@fresh_42 I just edited my post, does it fix the problem. I am not sure I can just do what you suggest, that since ##d## is a gcd, then it is an unit.
No. It is only a common factor of all coefficients. You can use this factor as ##g(x)=d## in definition 1. It then follows that ##d## is a unit since ##f## is primitive (def 1). You don't need the ##c_i## and the equation ##dc_i-c_i=0## doesn't hold.

elias001 said:
I try to just show that ##d=1##according to definition 2. I mean that is the requirement for ##f## to be a primitive polynomial?
Yes, but this is nitpicking. If ##d## is a unit in ##R,## which it is by definition 1, then there is an element ##d^{-1}\in R.## Now ##d\cdot d^{-1}## also divides all coefficients, so ##\operatorname{gcd}S=d\cdot d^{-1}=1.##

Making that divisor ##1## is just a technical trick.

The other direction is the difficult one.

Say ##f## is primitive (def 2). Then we assume that ##g(x)\,|\,f(x).## Hence, there is a polynomial ##h(x)\in R[x]## such that ##f(x)=g(x)\cdot h(x).##

Now, you must show that ##g(x)## is a unit in ##R,## i.e., that it is a constant polynomial, and the constant is invertible. How? We only know that the coefficients of ##f## have no common divisor, or the divisor ##1## to be exact. How can we use this to get information about ##g(x)##?
 
  • #13
@fresh_42 I still don't understand how the portion of Def 1 where it states: "...if the only constants that divides it are the units in..." allows me to infer ##d## is an unit? There are might be other elements that divides ##f(x)## that are not units. I want to get this subtle point sorted out before doing (b).
 
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  • #14
elias001 said:
@fresh_42 I still don't understand how the portion of Def 1 where it states: "...if the only units that divides it are the units in..." allows me to infer ##d## is an unit? There are might be other elements that divides ##f(x)## that are not units.
Are you sure that is exactly so in the book?

elias001 said:
##\textbf{Definition 1:}## Let ##R## be an UFD. A nonzero polynomial in ##R[x]## is said to be ##\textbf{primitive}## if the only units that divides it are the units in ##R##.
I think it should be ...
elias001 said:
##\textbf{Definition 1:}## Let ##R## be an UFD. A nonzero polynomial in ##R[x]## is said to be ##\textbf{primitive}## if the only factors that divides it are the units in ##R##.
... since I do not see that units in ##R## differ from the units in ##R[x]## which would make it an empty statement.
 
  • #15
@fresh_42 I attached a screenshot. Sorry, I made a typo it should say:"...if the only constants that divides it are the units in...". I edited post 1 in Def 1 also to reflect it. Still it doesn't change the fact that I still need to show that ##d## is an unit.


Screenshot_20250723_193249_FBReader Premium.webp
 
  • #16
elias001 said:
@fresh_42 I attached a screenshot. Sorry, I made a typo it should say:"...if the only constants that divides it are the units in...". I edited post 1 in Def 1 also to reflect it. Still it doesn't change the fact that I still need to show that ##d## is an unit.


View attachment 363639
##d## is a unit because definition 1 says so. We have ##d\,|\,f(x)## and definition 1 says that only units divide ##f.##
 
  • #17
@fresh_42 I should read that portion of Def 1 as: if ##x|f(x)## and ##x## is a constant, then ##x## is an unit?

I find the notion of primitive polynomial to be not very intuitive and i am having a hard time relating units of a polynomial to the gcd of its coefficients.

I asked one of the LLMs for an explanation, and here is what it says:

Content of a Polynomial: The GCD of the coefficients of a polynomial ##f(x)∈F[x]## is called the content of the polynomial, denoted ##\text{cont}(f).## If ##\text{cont}(f)=d ##, then ##f(x) = d \cdot g(x) ,## where ##g(x)\in F[x]## is a polynomial whose coefficients have GCD ##1## (i.e., ##g(x)## is primitive).

Relation to Units: The content ##\text{cont}(f)## is only defined up to a unit in ##F##. If ##d## is the GCD of the coefficients, then any other element ##d' = u \cdot d## , where ##u∈F^*## (the group of units in ##F##), is also a valid GCD. This is because, in an UFD, if ##d## divides each coefficient, so does ##u \cdot d ##, and vice versa, since ##u## is invertible.

For example:

In ##\mathbb{Z}[x]## , the units are ##\pm 1##, so the GCD of the coefficients is unique up to sign (e.g., ##\text{gcd}⁡(2,4)=2## or ##−2##).

In a field like ##\mathbb{Q}## , all non-zero elements are units, so the content of a polynomial is less significant, as any non-zero constant can be factored out and absorbed into a unit.

Implications for Factorization: The relationship is crucial in Gauss's lemma, which states that for a UFD ##F ##, the content of a product of polynomials satisfies ##\text{cont}(f \cdot g) = \text{cont}(f) \cdot \text{cont}(g)## (up to units). This implies that if a polynomial has content that is a unit in ##F##, it is primitive, and primitivity is preserved under multiplication in ##F[x]##.


Let ##f(x) = a_n x^n + \cdots + a_0 \in F[x]## be a polynomial, where ##a_i \in F.## The content of ##f,## denoted ##\text{cont}(f),## is defined as: ##\text{cont}(f) = \gcd(a_n, a_{n-1}, \ldots, a_0),## where the GCD is taken in the UFD ##F.## In an UFD, the GCD exists and is unique up to multiplication by units. An unit in ##F## is an element ##u \in F## such that there exists ##v \in F## with ##uv=1,## where ##1## is the multiplicative identity in ##F.## The set of units in ##F## is denoted ##F^*.##

Saying that ##\text{cont}(f)## is "defined up to a unit in ##F##" means: If ##d \in F## is a GCD of the coefficients ##\{a_n, \ldots, a_0\}## , then any other element ##d' \in F## of the form ##d' = u \cdot d## , where ##u \in F^*##, is also a GCD of the coefficients.
Conversely, if ##d'## is another GCD of the coefficients, then there exists a unit ##u \in F^*## such that ##d' = u \cdot d##.

In mathematical notation, this can be expressed as

##\text{cont}(f) \sim d \quad \text{if and only if} \quad d \text{ divides } a_i \text{ for all } i, \text{ and any other } d' \text{ with this property satisfies } d' = u \cdot d \text{ for some } u \in F^*.##
Equivalently, the content is an element of the quotient monoid ##\frac{F}{F^*}## , where two elements ##d, d' \in F## are equivalent if ##d' = u \cdot d## for some ##u \in F^*## .

Does the explanation appears to be ok/clear to you?
 
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  • #18
elias001 said:
@fresh_42 I should read that portion of Def 1 as: if ##x|f(x)## and ##x## is a constant, then ##x## is an unit?
Not really. It says, that if ##f(x)=a_0+a_1x+\ldots+a_nx^n## and ##d\,|\,a_0,a_1,\ldots,a_n## then ##d=1.##
It doesn't say anything about ##x.## The polynomial ##f(x)=x## is primitive, although ##x\,|\,f(x)## and ##x## isn't constant.

Let me quote how my book (van der Waerden) defines it.

The underlying ring ##R## is an integral domain (commutative and without zero divisors) with ##1.## The greatest common divisor ##d## of ##a_0,a_1,\ldots, a_n## is the content of ##f(x)=a_0+a_1x+\ldots+a_nx^n. ## If we take separate ##d## by writing
$$
f(x)=d\cdot g(x)
$$
then ##g(x)## has the content ##1.## Both, ##g(x)## and ##d## are uniquely determined up to unit factors. Polynomials of content ##1## are called primitive or unit-forms with respect to ##R.## van der Waerden uses the term unit-forms, but that is a bit outdated.

elias001 said:
I find the notion of primitive polynomial to be not very intuitive and i am having a hard time relating units of a polynomial to the gcd of its coefficients.

Whatever you call it, it is all about the greatest common divisor of the coefficients of ##f(x),## not ##f(x)## itself, for which we have the terms prime and irreducible. This greatest common divisor is a ring element, i.e. ##d\in R.## As such, it can be ##d=1## or ##d\neq 1.## Units are more or less irrelevant since we can always multiply any units without changing what it is all about. Units of ##R[x]## are even less relevant in this context. You are obsessed with units. Forget them. They are just irrelevant factors (in this context) and play the same role as ##1## does.

elias001 said:
I asked one of the LLMs for an explanation, and here is what it says:

You'd better not. You are already confused by using more than one book. I would concentrate on a single book and only ask questions about that specific book (whichever it is). LLMs tell you what they "think" you want to hear. They are worse than Wikipedia!

elias001 said:
Content of a Polynomial: The GCD of the coefficients of a polynomial ##f(x)∈F[x]## is called the content of the polynomial, denoted ##\text{cont}(f).## If ##\text{cont}(f)=d ##, then ##f(x) = d \cdot g(x) ,## where ##g(x)\in F[x]## is a polynomial whose coefficients have GCD ##1## (i.e., ##g(x)## is primitive).

Relation to Units: The content ##\text{cont}(f)## is only defined up to a unit in ##F##. If ##d## is the GCD of the coefficients, then any other element ##d' = u \cdot d## , where ##u∈F^*## (the group of units in ##F##), is also a valid GCD. This is because, in an UFD, if ##d## divides each coefficient, so does ##u \cdot d ##, and vice versa, since ##u## is invertible.

For example:

In ##\mathbb{Z}[x]## , the units are ##\pm 1##, so the GCD of the coefficients is unique up to sign (e.g., ##\text{gcd}⁡(2,4)=2## or ##−2##).

In a field like ##\mathbb{Q}## , all non-zero elements are units, so the content of a polynomial is less significant, as any non-zero constant can be factored out and absorbed into a unit.

Implications for Factorization: The relationship is crucial in Gauss's lemma, which states that for a UFD ##F ##, the content of a product of polynomials satisfies ##\text{cont}(f \cdot g) = \text{cont}(f) \cdot \text{cont}(g)## (up to units). This implies that if a polynomial has content that is a unit in ##F##, it is primitive, and primitivity is preserved under multiplication in ##F[x]##.

Let ##f(x) = a_n x^n + \cdots + a_0 \in F[x]## be a polynomial, where ##a_i \in F.## The content of ##f,## denoted ##\text{cont}(f),## is defined as: ##\text{cont}(f) = \gcd(a_n, a_{n-1}, \ldots, a_0),## where the GCD is taken in the UFD ##F.## In an UFD, the GCD exists and is unique up to multiplication by units. An unit in ##F## is an element ##u \in F## such that there exists ##v \in F## with ##uv=1,## where ##1## is the multiplicative identity in ##F.## The set of units in ##F## is denoted ##F^*.##

Saying that ##\text{cont}(f)## is "defined up to a unit in ##F##" means: If ##d \in F## is a GCD of the coefficients ##\{a_n, \ldots, a_0\}## , then any other element ##d' \in F## of the form ##d' = u \cdot d## , where ##u \in F^*##, is also a GCD of the coefficients.
Conversely, if ##d'## is another GCD of the coefficients, then there exists a unit ##u \in F^*## such that ##d' = u \cdot d##.

In mathematical notation, this can be expressed as

##\text{cont}(f) \sim d \quad \text{if and only if} \quad d \text{ divides } a_i \text{ for all } i, \text{ and any other } d' \text{ with this property satisfies } d' = u \cdot d \text{ for some } u \in F^*.##
Equivalently, the content is an element of the quotient monoid ##\frac{F}{F^*}## , where two elements ##d, d' \in F## are equivalent if ##d' = u \cdot d## for some ##u \in F^*## .

Does the explanation appears to be ok/clear to you?

That's more or less exactly how van der Waerden defined it. Again, forget about units. They are only a pollution in uniqueness statements.

It is simple. How would you factor the integer ##6##? You would probably answer ##6=2\cdot 3.## This shows that ##6## is reducible. It is also not a prime, since the two terms coincide for integers. However, the reason why ##6## isn't prime is a different one. If ##6\,|\,3\cdot 4## then we cannot conclude ##6\,|\,3## or ##6\,|\,4.## That is why ##6## isn't prime.

Now, what if I were to say my answer to that question is ##6=(-2)\cdot (-3)?## This is certainly true, and, what is more important, wouldn't change any of my previous arguments. We would only need to carry the minus sign through the arguments. ##-2## is a prime number! This is because ##-1## is a unit, and it shows that uniqueness is only given up to factors ##\pm 1,## the units in ##\mathbb{Z}.## And if the ring were a field, we would have many more units. That is why ##\mathbb{Q}## doesn't have any prime elements.
 
  • #19
@fresh_42 for (b)

Assumed fact: every polynomial ##m(x)=pq(x)## where ##p## is the gcd of the coefficients of ##m(x)## and the polynomial ##q(x)## is primitive.

Let ##f(x)## be a primitive polynomial with ##\text{content}(f)=1.## Why do/can I suppose that ##g(x)|f(x)?##.

If ##g(x)|f(x)## implies ##f(x)=g(x)h(x)## then the ##\text{content}(f)## is the same as the content of ##h(x),## hence ##h(x)## is primitive. By the assumed fact, ##f(x)=g(x)h(x)=\text{content}(f)h(x).##. This implies ##g(x)=\text{content}(f)=1.##

Here: "the ##\text{content}(f)## is the same as the content of ##h(x),## hence ##h(x)## is primitive." I want to use one more fact that ##\text{content}(f)=\text{content}(gh)=\text{content}(g)\text{content}(h),## then ##1=\text{content}(g)\text{content}(h)## implying ##\text{content}(g)## is an unit, so ##g(x)## is primitive. But then I don't know how to conclude that "the ##\text{content}(f)## is the same as the content of ##h(x),## hence ##h(x)## is primitive.". Maybe I can do the same argument like with ##g(x)##.

By the way, I now know why Def 1 is troubling me. If we are working in ##\Bbb{Q}[x]##, the polynomial ##3x^2+9x+15## has gcd of its coefficients equals ##3##, but that doesn't satisfy Def 2 since it requires gcd to be ##1##.
 
  • #20
elias001 said:
@fresh_42 for (b)

Assumed fact: every polynomial ##m(x)=pq(x)## where ##p## is the gcd of the coefficients of ##m(x)## and the polynomial ##q(x)## is primitive.

Let ##f(x)## be a primitive polynomial with ##\text{content}(f)=1.## Why do/can I suppose that ##g(x)|f(x)?##.
We have to. We know that ##f(x)## is primitive (def 2). Now, if we want to show ##f(x)## is primitive (def 1), we have to show what definition 1 states. The statement is, if ##g(x)\,|\,f(x)## then ##g(x)\in R^0.## Hence, we must assume that ##g(x)\,|\,f(x)## since these are the only relevant cases. All others don't matter.

elias001 said:
If ##g(x)|f(x)## implies ##f(x)=g(x)h(x)## then the ##\text{content}(f)## is the same as the content of ##h(x),## hence ##h(x)## is primitive. By the assumed fact, ##f(x)=g(x)h(x)=\text{content}(f)h(x).##. This implies ##g(x)=\text{content}(f)=1.##
Let me sort this out. We know that ##f(x) ## is primitive (def 2), i.e., the content of ##f(x)## is ##1.## We also know that ##f(x)=g(x)\cdot h(x).## What we need next is that the content of a product is the product of the contents of the factors. This should be proven. Given that we have
$$
1=\operatorname{content}(f(x))=\operatorname{content}(g(x))\cdot \operatorname{content}(h(x)).
$$
But this is an equation in ##R,## so ##\operatorname{content}(g(x))## and ##\operatorname{content}(h(x))## are units by definition of a unit. Hence, ##g(x)## and ##h(x)## are primitive (def 2). The contents are not exactly ##1## but units, which is good enough. If ##g(x)=d\in R## then ##d\in R^0## which proves that ##f(x)## is primitive (def 1).

elias001 said:
Here: "the ##\text{content}(f)## is the same as the content of ##h(x),## hence ##h(x)## is primitive." I want to use one more fact that ##\text{content}(f)=\text{content}(gh)=\text{content}(g)\text{content}(h),## then ##1=\text{content}(g)\text{content}(h)## implying ##\text{content}(g)## is an unit, so ##g(x)## is primitive. But then I don't know how to conclude that "the ##\text{content}(f)## is the same as the content of ##h(x),## hence ##h(x)## is primitive.". Maybe I can do the same argument like with ##g(x)##.

By the way, I now know why Def 1 is troubling me.
Me, too. I made a mistake. It is really what you said. It has to be

definition 1: ##f(x)## is primitive iff ## d\,|\,f(x) \wedge d\in R \Longrightarrow d\in R^0.##

Since ##x+x^2## is primitive, and ##x\,|\,(x+x^2)## we cannot conclude ##x\in R^0.## We must demand ##R\ni d\,|\,f(x).##

elias001 said:
If we are working in ##\Bbb{Q}[x]##, the polynomial ##3x^2+9x+15## has gcd of its coefficients equals ##3##, but that doesn't satisfy Def 2 since it requires gcd to be ##1##.
No. If we are working in ##\mathbb{Q}[x]## then any non-zero number is a unit. There is no greatest common divisor since all numbers are divisors, and up to units it is ##1## which is as good as any other non-zero number.

Your argument is only valid in ##\mathbb{Z}[x],## in which case ##3x^2+9x+15## isn't primitive whether you use definition 1 or 2.

That is why van der Waerden added "... primitive with respect to ##R##" and why I repeat the mantra of saying which ring you refer to.
 
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  • #21
@fresh_42 for (b) how do I put it all together. I am a little confused over our back and forth as to what can be used and what I need to do more justifications. I am using two facts:

Fact 1. Every polynomial in a polynomial ring can be written as a product of its content and a primitive polynomial.

Fact 2. ##\text{content}(f(x))=\text{content}(g(x))\cdot\text{content}(h(x)).##

So if I have ##f(x)=g(x)h(x)\quad (*)##, and we also have ##f(x)=\text{content}(f)h(x)\quad (**)##, then ##f(x)=g(x)h(x)=\text{content}(f)h(x),## and ##\text{content}(f)=1##, so in ##(**)## I can conclude that ##h(x)## is primitive from Fact 1? Also since ##\text{content}(f)=g(x)=1##, then I can also conclude that ##g(x)## is also primitive?

let's leave the issue of Def 1 with the issues of contestants and units for now until after (b) is resolved, since i think that might affect whether the argument in (a) need to be modified to take into account which ring one is speaking about.
 
  • #22
elias001 said:
@fresh_42 for (b) how do I put it all together. I am a little confused over our back and forth as to what can be used and what I need to do more justifications.
Sorry, my bad. Let's clean up the mess a bit.
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We start with ##R## being a UFD and particularly an integral domain, so no zero divisors except ##0.## The units in ##R## will still be denoted as ##R^0.##
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Definiton 1: ##f(x)\in R[x]## is primitive iff ##\left[d\,|\,f(x)\wedge d\in R\Longrightarrow d\in R^0\right].##

Definiton 2: ##f(x)=a_0+a_1x+\ldots+a_nx^n\in R[x]## is primitive iff ##\operatorname{gcd}(a_0,\ldots,a_n)=1.##
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a) Definition 1 ## \Longrightarrow ## Definition 2:

Let ##d=\operatorname{gcd}(a_0,\ldots,a_n).## Then ##d\in R## and ##d\,|\,f(x).## By definition 1, this implies that ##d\in R^0.## Thus, it is invertible, i.e., ##d\cdot d^{-1}=1## with ##d^{-1}\in R^0.## Finally, if ##d=\operatorname{gcd}(a_0,\ldots,a_n)## then so is ##1=d\cdot d^{-1}=\operatorname{gcd}(a_0,\ldots,a_n)## proving definition 2.

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b) Definition 2 ## \Longrightarrow ## Definition 1:

To prove definition 1, we need an element ##d\,|\,f(x)## with ##d\in R.## This means that ##d## is a divisor of all coefficients of ##f(x).## Therefore, ##d## divides the greatest of all common divisors, i.e.,
$$
d\,|\,\operatorname{gcd}(a_0,\ldots,a_n)=1
$$
by definition 2. But if ##d\,|\,1## then it is an element from ##R^0## by definition of units. If ##d\in R^0## then this is exactly what definition 1 requires to be shown.
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Fact 1 is simply the distributive law in ##R.## Fact 2 seems obvious, but I suggest you try to prove it formally.
 
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  • #23
@fresh_42 The way you rewrote (a) accounts for different polynomial rings: ##\Bbb{Q}[x], \Bbb{Z}[x].##

For Fact 1, Hungerford gave a weird direct proof. Screenshots:
Screenshot_2025-07-24-13-01-12-033.webp
Screenshot_2025-07-24-13-01-27-985.webp


Here is a proof by contradiction from a YT video

Can I ask you about the other post about that localization example?
 

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  • #24
elias001 said:
Can I ask you about the other post about that localization example?
Sure, but do it there or even better in a new thread.
 
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