Friday, 23 June 2017

abstract algebra - Showing that xn2 is irreducible in mathbbQ[X]



I'm trying to show that the polynomial Xn2 (nN) is irreducible in Q[X] but am a bit stuck. Methods I know to show irreducibility:





  • Gauss' lemma - which says that if I can show it is irreducible in Z[X] then it will be irreducible in Q[X].

  • This would allow me to check reduction modulo a prime.



However this doesn't work in this case, because if I reduce mod 2, then the polynomial is just Xn which is reducible.



I'm also aware of Eisensteins criterion where I can check that if a prime divides all the coefficients but its square doesn't divide the constant coefficient, then it is irreducible.



None of these methods are working for this polynomial so help would be much appreciated! Also are there any general methods to look out for when trying to show polynomials are irreducible?




Thanks


Answer



Let P(x)=xn2=(xk++a)(xnk++b);  1kn1.


On other hand P(x)=(xn2ε0)(xn2ε1)(xn2εn1),

where εi=e2πin,i=0,1,,n1. Therefore a=(1)kn2kεi0εi1εik1.
It's easy to see that a can't be integer.


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