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Thursday, October 10, 2013

Characterization of Finite Subfield Structure (13.5.3-4)

Dummit and Foote Abstract Algebra, section 13.5, exercises 3-4:

MathJax TeX Test Page 3. Prove d | nxd1 | xn1 in Z[x].

4. Let 1<aZ. Prove for positive n,dZ that d | nad1 | an1. Conclude in particular that FpdFpnd | n.

Proof: (3) () Evidently (xd1)(xnd+xn2d+...+xd+1)=xn1. () Write f(x)(xd1)=xn1 and assume d  n so n=qd+r for some 0<r<d. We thus have every solution of xd1 is a solution of xn1 in any field containing Z. Let ζ be a primitive dth root of unity. Then ζn1=ζqd+r1=ζr10, a contradiction.

(4a) First we note that an1ar1 mod ad1 if nr mod d. This is because, after writing n=qd+r for 0r<d, we have 1(ad)qaqdanr and now anar. Since 0ar1<ad1 we have an10 if and only if n0 mod d.

(4b) Assume d | n; then pd1 | pn1 so xpd11 | xpn11 and the splitting field of the latter contains the former, i.e. FpdFpn. Assume FpdFpn; then we see these are exactly the splitting fields of xpd11 and xpn11 respectively, andxpd11=αFpd(xα) | αFpn(xα)=xpn11implying pd1 | pn1 implying d | n. 

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