Results 1 to 7 of 7
Like Tree2Thanks
  • 1 Post By Deveno
  • 1 Post By Deveno

Math Help - Polynomial Rings over Fields

  1. #1
    Super Member Bernhard's Avatar
    Joined
    Jan 2010
    From
    Hobart, Tasmania, Australia
    Posts
    558
    Thanks
    2

    Polynomial Rings over Fields

    Let F be a field.

    Let f(x) be a polynomial in F[x].

    Prove that F[x]/(f(x)) is a field if and only if f(x) is irreducible.

    Would appreciate help with this problem.

    Peter
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor

    Joined
    Mar 2011
    From
    Tejas
    Posts
    3,312
    Thanks
    693

    Re: Polynomial Rings over Fields

    show if f(x) is irreducible, (f(x)) is a maximal ideal.

    since every polynomial ring over a field F is a euclidean domain, it is a principal ideal domain.

    every euclidean domain is principal:

    suppose R is euclidean, and I a non-zero ideal of R. let u in R be such that d(u) is minimal.

    given ANY x in I, we can write x = qu + r, where either d(r) < d(u), or r = 0.

    now r = x - qu, which is in I, since x is in I, and u is in I, so qu is in I, so x - qu is in I, that is: r is in I.

    since d(u) is minimal, we must have r = 0. thus x = qu, which is to say x is in (u), so I is contained in (u).

    but clearly (u) is contained in I (since u is IN I). thus I = (u), that is: any ideal is principal.

    so in F[x], any ideal is of the form I = (f(x)) for some polynomial f(x).

    so suppose I ⊆ J in F(x). since I = (f(x)) and J = (g(x)), this means that f(x) = r(x)g(x), for some r(x) in F[x], that is: g(x)|f(x).

    if f(x) is irreducible, r(x) must be a unit, or g(x) must be a unit.

    if r(x) is a unit, (f(x)) = (g(x)), so I = J.

    if g(x) is a unit, J = F[x], thus I is maximal.

    if I is a maximal ideal of R (a commutative ring with unity), R/I is a field:

    suppose K is an ideal of R/I. since we have the surjective ring-homomorphism φ: R-->R/I, it follows that φ-1(K) is an ideal of R containing I. since I is maximal, either φ-1(K) = I, or φ-1(K) = R.

    in the first case: K = φ(φ-1(K)) = φ(I) = {0}.

    in the second case, K = φ(φ-1(K)) = φ(R) = R/I. the fact that R/I is a field comes from:

    if R is a commutative ring with unity whose only ideals are {0} and R, then R is a field:

    suppose x in R is non-zero. then (x) is an ideal of R. since (x) ≠ {0}, (x) = R, thus 1 is in (x), hence 1 = xu, for some u in R, so that x is a unit.

    ********

    on the other hand, suppose F[x]/(f(x)) is a field. then (f(x)) must be a maximal ideal of F[x] (for if not, F[x]/(f(x)) would contain other ideals besides itself and {0}, which is impossible, since in a field, every non-zero element is a unit).

    so suppose we have f(x) = g(x)k(x), for some g(x), k(x) in F[x]. then g(x)|f(x). hence (f(x)) ⊆ (g(x)). since f(x) is a maximal ideal of F[x], either:

    a) (g(x)) = F[x], in which case 1 is in (g(x)), so 1 = g(x)h(x), for some h(x) in F[x], that is: g(x) is a unit.

    b) (g(x)) = (f(x)) in which case g(x) = f(x)h(x), so we get:

    f(x) = g(x)k(x) = f(x)h(x)k(x).

    since F[x] is an integral domain (every principal ideal domain is), we have cancellation, so 1 = h(x)k(x), so that k(x) is a unit.

    so if f(x) = g(x)k(x), one of g(x) or k(x) must be a unit, that is: f is irreducible.
    Thanks from Bernhard
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Super Member Bernhard's Avatar
    Joined
    Jan 2010
    From
    Hobart, Tasmania, Australia
    Posts
    558
    Thanks
    2

    Re: Polynomial Rings over Fields

    You write: "suppose R is euclidean, and I a non-zero ideal of R. let u in R be such that d(u) is minimal."

    I am assuming that you are using the Well Ordering of Z (actually of  Z^+ \cup \{0\} and just taking the minimum of the d(x)?

    Is that correct?

    Peter
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Super Member Bernhard's Avatar
    Joined
    Jan 2010
    From
    Hobart, Tasmania, Australia
    Posts
    558
    Thanks
    2

    Re: Polynomial Rings over Fields

    I have followed your proof down to the point where you state:

    "if g(x) is a unit, J = F[x], thus I is maximal."

    How do we know this? That is how do we know there is no ideal K such that  I \subset K \subset J ?

    Can you please clarify how J = F[x] implies that I is maximal.

    Peter
    Follow Math Help Forum on Facebook and Google+

  5. #5
    MHF Contributor

    Joined
    Mar 2011
    From
    Tejas
    Posts
    3,312
    Thanks
    693

    Re: Polynomial Rings over Fields

    a maximal ideal I of an ideal R is one such that:

    if J is an ideal with I ⊆ J, either J = I, or J = R (in other words a maximal ideal doesn't have any other ideals properly containing it except R).

    we have shown that for any ideal I generated by an irreducible f(x), if J is an ideal containing I:

    J = I, or J = R (which is in this case, F[x]).

    that is: there aren't any ideals between (f(x)) and the entire ring, F[x].

    another way to see this is:

    F[x] is a euclidean domain. euclidean domains are also PIDs. PIDs are UFDs. in a UFD, irreducible = prime.

    thus (f(x)) is an ideal generated by a prime element. ideals generated by a prime element are prime ideals. in a PID, prime ideals are maximal.

    ***************

    it's not J = F[x], that implies I is maximal, it's

    J = F[x] or J = I, that implies I is maximal.
    Follow Math Help Forum on Facebook and Google+

  6. #6
    Super Member Bernhard's Avatar
    Joined
    Jan 2010
    From
    Hobart, Tasmania, Australia
    Posts
    558
    Thanks
    2

    Re: Polynomial Rings over Fields

    In your proof above you write: "suppose R is euclidean, and I a non-zero ideal of R. let u in R be such that d(u) is minimal."

    I am assuming that you are using the Well Ordering of Z and just taking the minimum of the d(x)?

    Is that correct?

    Peter
    Follow Math Help Forum on Facebook and Google+

  7. #7
    MHF Contributor

    Joined
    Mar 2011
    From
    Tejas
    Posts
    3,312
    Thanks
    693

    Re: Polynomial Rings over Fields

    the well-ordering of N. the degree of a non-zero polynomial is non-negative.
    Thanks from Bernhard
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Polynomial Rings over Fields
    Posted in the Advanced Algebra Forum
    Replies: 1
    Last Post: December 30th 2012, 07:21 PM
  2. Replies: 5
    Last Post: December 28th 2012, 10:27 PM
  3. Polynomial Rings over fields with characteristic p
    Posted in the Advanced Algebra Forum
    Replies: 1
    Last Post: May 25th 2012, 11:01 AM
  4. Rings, ID, and Fields
    Posted in the Advanced Algebra Forum
    Replies: 2
    Last Post: March 24th 2010, 09:31 AM
  5. Rings and Fields
    Posted in the Advanced Algebra Forum
    Replies: 1
    Last Post: March 22nd 2009, 05:44 PM

Search Tags


/mathhelpforum @mathhelpforum