Wednesday, 3 February 2016

abstract algebra - mathbbQ(sqrtp1+sqrtp2)=mathbbQ(sqrtp1,sqrtp2) for p1,p2 primes.



I want to prove that Q(p1+p2)=Q(p1,p2) for p1,p2 primes. I know this was proved before for the generalized case of p1,....,pn primes here:



How to prove that Q[p1,p2,,pn]=Q[p1+p2++pn], for pi prime?



And every proof of this kind of equality involves Galois theory but I still don't know Galois theory and I'm supposed to prove this with only basic Field theory(field extensions, irreducible polynomials, algebraic extensions, etc).



By definition Q(p1+p2) is the smallest field containing Q and p1+p2, also Q(p1,p2) is the smallest field containing p1 and p2. Or




Q(p1,p2)={a+bp1+cp2+dp1p2a,b,c,dQ}



Q(p1+p2)={a+b(p1+p2)a,bQ}



Also I know that [Q(p1,p2):Q]=4 from this:



Proving that (Q[p1,,pn]:Q)=2n for distinct primes pi.



Still don't know how to proceed proving this two extensions are the same.



Answer



Hint



Step 1 :
p1p2=p1p2p1+p2Q(p1+p2)



Step 2 :



p1=(p1+p2)+(p1p2)2Q(p1+p2).




I let you manage to show that p2Q(p1+p2) as well and conclude the equality.


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