Saturday, 23 June 2007

number fields - Given an integer n and a finite extension K of Q , find a polynomial of degree n that is irreducible over K

Here is a more general result.



Theorem: Let (K,||) be a non-Archimedean normed field with completion hatK. Let mathcalL/hatK be a finite separable extension of degree d. Then there exists a degree d separable field extension L/K such that LhatK=mathcalL.



In particular, as long as the completion of K admits a separable field extension of a certain degree d, so does K itself, necessarily of the form K[t]/(P(t)) by the primitive element theorem. Moreover, as long as K has characteristic zero and carries a nontrivial discrete valuation, it admits finite separable extensions of all finite degrees.



For a proof of this theorem using Krasner's Lemma, see Section 3.5 of



http://math.uga.edu/~pete/8410Chapter3.pdf



When the norm corresponds to discrete valuation v (e.g. ||=||mathfrakp
the mathfrakp-adic norm for a prime ideal mathfrakp of a number field K) one can get away with less: by weak approximation, there exists alphainK with v(alpha)=1. For any positive integer n prime to the characteristic of K, by Eisenstein's Criterion the polynomial tnalphainK[t] is (separable and) irreducible even over the completion hatK, so is certainly irreducible over K.

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