Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Why is Q(sqrt(2)) not isomorphic with Q(sqrt(3))

  1. Mar 2, 2008 #1
    1. The problem statement, all variables and given/known data

    why is Q[[tex]\sqrt{2}[/tex]] is not isomorphic to Q[[tex]\sqrt{3}[/tex]]?

    2. Relevant equations

    3. The attempt at a solution

    I do not know where to start?
  2. jcsd
  3. Mar 2, 2008 #2
    Assume you have an isomorphism [tex]f[/tex] from [tex]\mathbb{Q}[\sqrt{2}][/tex] to [tex]\mathbb{Q}[\sqrt{3}][/tex]. Think about what [tex]\sqrt{2}[/tex] can be mapped to. Hint: you can deduce that [tex]f(\sqrt{2})^2 = 2[/tex]
  4. Mar 2, 2008 #3
    Thanks for the help.
    If I assume there is an isomorphism from [tex]\mathbb{Q}[\sqrt{2}][/tex] to [tex]\mathbb{Q}[\sqrt{3}][/tex] then since [tex]\sqrt{2}[/tex] is not rational, it can only be mapped to [tex]\sqrt{3}[/tex]. This says that (a+b[tex]\sqrt{2}[/tex]) is mapped to (a+b[tex]\sqrt{3}[/tex]) but the map here is not a homomorphism since it fails this requirement: f(ab) = f(a)f(b).
    I am just curious if there is some characteristic that [tex]\mathbb{Q}[\sqrt{2}][/tex] but [tex]\mathbb{Q}[\sqrt{3}][/tex] does not? ( I guess not?)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook