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Number of Homomorphisms

  1. Nov 2, 2011 #1

    I am trying to calculate the number of homomorphisms from one field to another:

    a) F2 ---> F3
    b) Q[X]/(X7 - 3) ---> Q[X]/(X8 + 4X5 - 6X + 2)
    c) F7 [X] / (X2 + X - 1) ---> F7[X] / (X2 + 1)
    d) Q( 21/4 ) ---> C

    Attempt at a solution

    a) I'm pretty sure there are no homomorphisms between F2 and F3 because if there was a homomorphism f, then f(1+1) = f(0) which does not equal f(1) +f(1) = 2

    b) I think I need to see how many roots there are of X7 - 3 in Q[X]/(X8 + 4X5 - 6X + 2) since there is a bijection between that and the number of homomorphisms?

    c) Similarly here

    d) For this one I think the answer is four (I'm really not sure) because there is a bijection between K-Homomorphisms and the roots of the minimal polynomial of 21/4 in C, which would be 4. And over fields K-homomorphisms are ring homomorphisms?

    In all honesty I am pretty stuck, and if anyone could give me any advice that would be fantastic.

    Thanks in advance.
    Last edited: Nov 2, 2011
  2. jcsd
  3. Nov 2, 2011 #2
    a) I agree
    b) I am thinking that a homomorphism f will be uniquely defined by f(x)
    Prove it.
    But 0 = f(0) = f(x^7 - 3) = f(x)^7 - f(3) = f(x)^7 - 3
    and how many solutions does this have?
    But I suppose we have to bear in mind that this zero is k(x^8 + 4x^5 - 6x + 2) for any k
    c) similar
    d) Using this same method would seem to imply the identity is the only homomorphism, but there are 4 K-Homomorphisms as you said, so we are in trouble here

    This is still a work in progress for me but I hope it helps
    Last edited: Nov 2, 2011
  4. Nov 2, 2011 #3
    Thanks meandonlyme ,

    for b) could I put x7 - 3 into Q[X]/(X8 + 4X5 - 6X + 2)
    (which is x7 - 3 still) and then calculate the number of roots in there: 1 since it is over Q.

    So there is one root and hence one homomorphism.

    Similarly for c) and d)
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