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Homework Help: Gaussian Norms

  1. Oct 23, 2013 #1
    1. The problem statement, all variables and given/known data
    Suppose p is a prime number. Prove that p is irreducible in Z[√−5] if and only if there does not exist α ∈ Z[√−5] such that N(α) = p.
    Using this, find the smallest prime number that is not irreducible in Z[√−5].

    2. Relevant equations
    α = a+b√−5 ∈ Z[√−5]
    N(α) = a2 + 5b2
    N(α)N(β) = N(αβ)

    3. The attempt at a solution
    I did => so I'm now doing <=

    (Contraposition)Suppose that p is reducible in Z√-5 and isn't prime, then we know that p can be a product of two numbers: call them x,y ∈ Z√-5. Then we get that N(P)=n(x,y)=n(x)n(y)=(a2+5b2)(c2+5d2)

    then i have no idea what to do
  2. jcsd
  3. Oct 23, 2013 #2


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    Science Advisor

    You seem to be confused as to what "<=" is. The fact that p is prime is a given for either direction. You can't say "Suppose p isn't prime". The converse is "If there does not exist [itex]a\in Z[\sqrt{-5}][/itex] such that N(a)= p, then p is irreducible in [itex]Z[\sqrt{-5}][/itex]."
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