How to prove this ideal is a prime ideal

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It's not a prime ideal. $(x-1)(x^2-x-1)\in I$, while $x-1\not\in I$ and $x^2-x-1\not\in I$.

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yoyostein
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Updated on August 01, 2022

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  • yoyostein
    yoyostein over 1 year

    Let $I=\langle 3, f\rangle$ be an ideal in $\mathbb{Z}[X]$, where $f(x)=x^3+x^2+1$. How do I show that $I$ is a prime ideal?

    I know that $I$ is not a principal ideal, using an argument similar to: Why is $(3,x^3-x^2+2x-1)$ not principal in $\mathbb{Z}[x]$?

    I tried proving using definition, letting $ab\in I$ and trying to show that $a\in I$ or $b\in I$.

    Let $ab\in I$. Then $ab=3g+fh$ for some $g,h\in\mathbb{Z}[X]$.

    At this point, I have no idea what to do. I tried using the division algorithm to write $g=q_1f+r_1$, where $\deg r_1<3$, and $h=q_2f+r_2$, where $\deg r_2<3$.

    Then $ab=3q_1f+3r_1+f^2q_2+fr_2$. I am not sure if this is the right track, or it just complicates things further.

    Thanks for any help!


    I also had the idea of using: $I$ is a prime ideal iff $R/I$ is an integral domain, but I realised that it amounts to the same approach as above.

    • Michael L.
      Michael L. about 7 years
      Try showing the contrapositive: that for $a, b \in \mathbb{Z}[X]$, if $a, b \notin I$, then $ab \notin I$. Consider the remainders upon reducing $a$ and $b$ in terms of $3$ and $f$.
  • egreg
    egreg about 7 years
    Isn't it easier by noting that $f(1)=3$, so $f(x)-3=(x-1)(x^2+2x+2)\in I$?
  • Cave Johnson
    Cave Johnson about 7 years
    @egreg Thanks, you're quite right :) BTW, I'm now curious about the examples where $I$ is a prime ideal in $\mathbb{Z}[x]$ but not a principal ideal. Since it can be shown that if a prime ideal $\mathfrak p$ satisfies $\mathfrak{p}\cap\mathbb{Z}=\{0\}$, then $\mathfrak p$ is a principal ideal, so the example must be of the form $(n)+(f)$, where $n$ is a composite number. Do you have any idea?
  • egreg
    egreg about 7 years
    If $\mathfrak{p}$ is prime and $\mathfrak{p}\cap\mathbb{Z}=n\mathbb{Z}$, with $n>0$, then $n$ must be prime.
  • Cave Johnson
    Cave Johnson about 7 years
    @egreg I see. So there is actually no prime ideal in $\mathbb{Z}[x]$ which is not a principal ideal?
  • egreg
    egreg about 7 years
    Why? $(2,x)$ is prime and not principal.
  • Cave Johnson
    Cave Johnson about 7 years
    @egreg Unfortunately I mixed up the principal ideal and maximal ideal... Thanks a lot for clarification!
  • yoyostein
    yoyostein about 7 years
    Thanks a lot. What is the fastest way to see $(x-1), (x^2+2x+2)$ not in $I$?