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

Q(sqrt(2)) and Q(sqrt(3)) not isomorphic?

  1. Apr 11, 2009 #1
    Hello all,

    I am studying Algebra and in the chapter where Galois theory is introduced, I
    see the following exercise:

    "Prove that Q(sqrt(2)) and Q(sqrt(3)) are not isomorphic"

    Well, It seems that I am a bit behind because I really don't get it... :(
    I mean, I'm sure that this is the case, since it is a question in the book
    (and surely 'not' is not a typo!!), but these are vector spaces over Q,
    both of dimension 2, so shouldn't they be isomorphic by sending
    sqrt(2) to sqrt(3) and any rational number to itself?!

    What do I miss here?

    Thanks a lot in advance..
    Last edited: Apr 11, 2009
  2. jcsd
  3. Apr 11, 2009 #2
    Ooops! I think I see it now..
    They are isomorphic as vector spaces but not as fields, right?
    The isomorphism I said above does not respect the product..

    That's it, right?!
  4. Apr 11, 2009 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Right. (Have you yet shown there isn't a field isomorphism?)
  5. Apr 12, 2009 #4
    What more is true, is that given any square free intgers m and n, Q(sqrt(m)) and Q(sqrt(n)) are nonisomorphic. Intution serves right when you say that it "does not respect the product" but being more rigorous, show that no ismorphism can possibly exist between the two fields by first showing that any isomorphism fixes Q and that sqrt 2 (in this specific case) cannot be sent to any rational number, ie. sqrt 2 is sent to a+b*sqrt 3 for some nonzero rational b. This proof easily generalizes to square free m and n.
  6. Apr 12, 2009 #5
    Thanks a lot!!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook