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Compute the minimal polynomials over the field $\mathbb{Q}$ of the given numbers

  1. $\sqrt{2+i\sqrt{2}}$
  2. $\sqrt{1+ \sqrt{3}}$
  3. $5^\frac{1}{4}$
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Some of the observations made here math.stackexchange.com/questions/288036/… might be useful. –  Andreas Caranti Jan 27 '13 at 14:27
3  
How to ask a homework question –  Nate Eldredge Jan 27 '13 at 19:00

1 Answer 1

Partial answer:

Let $\alpha =\sqrt{2+i\sqrt{2}}$. We have $\displaystyle \alpha =\sqrt{2+i\sqrt{2}} \Longrightarrow \alpha ^2 = 2 +i\sqrt{2} \Longrightarrow \alpha^2-2=i\sqrt{2} \Longrightarrow \alpha ^4 -4\alpha ^2 + 4=-2$ $\displaystyle \Longrightarrow \alpha ^4 -4\alpha ^2 +6=0$

It follows that $\alpha$ is a root of the polynomial $\displaystyle m_\alpha(t):=t^4-4t^2+6\in \mathbb{Q} \textbf{[}t\textbf{]}$.

Could it be that $m_\alpha$ is irreducible over $\mathbb{Q}$? Ask Eisenstein.

The others are similar.

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So for the third one, I get x^4-5=0, and using Eisenstein's criterion for p=5 we can show that it is irreducible over Q, also it is primitive so it is minimal? right? –  Mathematician Jan 27 '13 at 14:37
    
@WaqasAliAzhar $x^4 - 5$ is the minimal polynomial of $\displaystyle 5^{\frac{1}{4}}$ because it is irreducible over $\mathbb{Q}$ because of Eisentein, because it is a monic polynomial and because $\displaystyle (5^{\frac{1}{4}})^4-5=0$. I don't see how primitive polynomials related to this. –  Git Gud Jan 27 '13 at 14:41
    
Isn't it enough to show some polynomial monic and irreducible to show it is minimal? –  Mathematician Jan 27 '13 at 14:44
    
@WaqasAliAzhar Yes, that's what I explained. –  Git Gud Jan 27 '13 at 14:45
    
Thank you ! Greatly appreciated –  Mathematician Jan 27 '13 at 14:45

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