Algebra 111C Assignment 4
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Algebra 111C
Assignment 4
SOLUTIONS TO SELECTED PROBLEMS
1. Let f (x) e F [x] be separable. Prove that f is irreducible over F if and only if for any ri , rk roots of f there exists φ e GalF (f) s.t. φ(ri ) = rk .
Solution.
Only if. Argue by contrapositive. Let f be not irreducible. That is, f is reducilble. Then by the Unique Factorization and separability we can write
f (x) = g(x)h(x) with some f, g e F [x], deg f, g > 0,
r1 e Sf is a root of h, rn e Sf is a root of g , rn is not a root of h. For any φ e GalF (f) = Aut(Sf /F) we must have h(r1 ) = 0 告 h(φ(r1 )) = 0 since h e F [x]. Hence φ(r1 ) rn for any such φ . Thus the negation of the transitivity holds.
If. This follows from the solution of the corresponding problem from the previous HW. The actual statement of the problem does not fit by itself. Fix r1 and r2 . By the corresponding theorem (see the previous HW) find fields isomorphism φ such that
F (r1 ) φ F (r2 )
, φ(r1 ) = r2 .
idF
Next, apply the isomorphism extension theorem to fields F (r1 ), F (r2 ), isomorphism φ between them, polynomial(s) f e F [x] - F (α1 )[x], φ * f = f e F [x] - F (α2 )[x] with their splitting field(s) Sf , Sφ * f = Sf to get the automorphism ψ such that
ψ
.
F (r1 ) φ F (r2 )
Combining two diagrams we get ψ e Aut(Sf /F) with ψ(r1 ) = r2 . 口
2. Let E be the smallest Galois extension of Q containing ^2 + ^32. Find |Gal(E/Q)|.
Identify a familiar group isomorphic to Gal(E/Q). For that use the table of all small groups from sec. 5.3 in DF. Prove your isomorphism.
Solution. Any extension must contain Q(^2 +^32). Then Q(^2 +^32) = Q(^2, ^32) by HW1 argument: α = (^2 +^32),
2 = ( ^32)3 = (α - ^2)3 = α2 - 3α2 ^2 + 6α - 2^2 告 ^2 e Q(^2 +^32),
告 ′32 e Q(′2 +′32).
Next, extension Q(′2, ′32)/Q is not Galois. Indeed, the degree [Q(′2, ′32) : Q] = 6.
At the same time x2 - 2, x3 - 2 e Q[x]. Hence any φ e Aut(Q(′2, ′32)/Q) must map the roots of the polynomials belonging to Q(′2, ′32) into the roots belonging to Q(′2, ′32). Thus any φ must map ′32 3| ′32, 士 ′2 3| 士 ′2. Hence
|Aut(Q(′2, ′32)/Q)| > 2 < 6 = [Q(′2, ′32) : Q].
Consequently any Galois extension E/Q must have
[E : Q(′2, ′32)] ~ 2, [E : Q] ~ 12.
One such Galois extension is the splitting field of separable polynomial S(x2 -2)(x3 -2) = Q(′2, ′32, i′3). Thus
Gal(E/Q) |
= 12 |
To find the (isomorphic) group use
Gal(S(x2 -2)(x3 -2) /Q) G < S5 .
Write the set of distinct roots {r1 , r2 , r3 , r4 , r5 } where the first three are roots of x3 - 2 and r4 ,5 are roots of x2 - 2. Any φ e G fixes the two sets. There are 6 permutations of r1 ,2 ,3 and 2 permutations of r4 ,5 , which give exactly 12 = |Gal(S(x2 -2)(x3 -2) /Q)|. Thus
G 4 σ 1 σ2 , σ 1 e Symr1 ,2 ,3 S3 , σ2 e Symr1 ,2 S2 .
We know that G must be one of the groups from the list. It’s enough to
guess to which one. Product of cycles α = (r1 r2 r3 )(r4 r5 ) e G has order 6,
α6 = 1.
Cycle (r1 r2 ) = (r1 r2 )(r3 )(r4 )(r5 ) e G has order 2,
β 2 = 1.
Moreover, αβ = (r1 r3 )(r2 )(r4 r5 ) and βα- 1 = (r1 r2 )(r3 r2 r1 )(r4 r5 ) = (r1 r3 )(r2 )(r4 r5 ), so
αβ = βα- 1 .
This gives
Gal(E/Q) D12 ,
the dihedral group of the regular 6-gone.
3. Let F be a number field. Let G be the Galois group of a separable f(x) e F [x], deg f = 3. Prove that G S3 or Z3 or Z2 . Here S3 = Sym51 ,2 ,3{
and Zm is the cyclic group of order m.
For f irreducible over F prove G S3 or Z3 .
Give some degree 3 polynomials over a number field with Galois groups isomorphic to S3 , Z2 . For Z3 see problem below.
Solution.
(a) The first part follows at once from HW2.
(b) Let f be separable and irreducible in F [x], deg f = 3. Identifying think G < S3 . The options for |G| are 1, 2, 3, 6. Then
|G| = 6 告 G = S3 ,
|G| = 3 告 G Z3 (3 is prime).
By the problem solved above G must act transitively on {1, 2, 3}: Ⅴi, j 刁φ e G : φ(i) = j . Hence |G| = 1 告 G = {1} is impossible. Similarly, if |G| = 2 then G = {1, σ}. Write the cycle decomposition for σ . It cannot be a 3-cycle since then |G| ~ 3. If it contains a cycle of order 2 then σ = (i1 i2 )(i3 ) and i3 cannot be mapped into i1 by G. Hence |G| = 2 is also impossible.
(c) Polynomial x3 -2 is separable and has the splitting field S = Q( ′32, i′3) (done in class). Since [S : Q] = 6 must have |Gal(S/Q)| = 6 and
Gal(S/Q) < S3 . Hence GalQ (x3 - 2) S3 .
GalQ (x(x2 + 1)) Z2 (why?)
口
4. For this problem you need to recall the notions from 111A related to sn, even permutations, transpositions,
. . . , and their properties, sec . 1 .3 and 3 .5 in DF . Also, for insights check sec . 14 .6 in DF .
Let f e F [x] be written in its splitting field as f(x) = (x - r1 )...(x -
discriminant of f, by
∆ = (ri - rj )2 .
1≤i<j<n
Define ρ = 1≤i<j<n(ri - rj ), ∆ = ρ2 .
(a) For polynomials x2 + ax + b and x3 + ax2 + bx + c over F , compute
∆ as a function of a, b, c. If you find the full cubic case too painful, just do 北3 + p北 + q . (b) Prove ∆ 0 if and only if f(x) is separable.
(c) Prove that ∆ e F . In general p F .
(d) Let ∆ 0. Prove that σ e Sn is an even permutation if and only if ρ(r1 , . . . , n) = ρ(rσ(1) , . . . , rσ(n)), the sign of ρ is not changed by
σ . Hint: permutation is even 兮 it equals an even number of transpositions . How does a single transposition change p?
(e) Let ∆ 0. Prove ρ e F if and only if every φ e GalF (f) gives an even permutation of roots. Use this to prove GalQ (x3 -3x2 +3) Z3 .
Solution. Items (a)-(e) are, for example, in Beachy-Blair, sec. 8.6.
2023-06-09