Algebra 111C Assignment 3
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Algebra 111C
Assignment 3
SOLUTIONS TO SOME PROBLEMS
1. Let F c E be number fields, [E : F] = 4. Prove that the following two conditions are equivalent:
(a) There exists intermediate F c K c E , [K : F] = 2;
(b) E = F (γ) for some γ with mγ,F (x) = x4 + ax2 + b e F [x].
Hint: to prove a →b first establish i = 长 (α) , α2 ∈ 长 , F = i(B) , B 2 ∈ i . Then consider cases B 2 长 and B 2 ∈ 长 separately. In the first case finding .B,长 and 长 (B) is straightforward . In the second case investigate 长 (y) for y = α(B + 1) .
Solution. The text below is lengthy only because I indicate alternative ways to prove the claims. The actual solution is not that long.
b ÷ a. We are given [F (γ) : F] = 4 and element γ 2 is a root of quadratic equation x2 + ax + b. Therefore [F (γ2 ) : F] = deg曰 γ 2 < 2. If
[F (γ2 ) : F] = 1
then γ 2 e F . In this case γ solves
x2 + = 0,
∈曰
which means [F (γ) : F] = deg曰 γ < 2 contradicting the assumption on F (γ).
Hence
F c F (γ2 ) c F (γ), [F (γ2 ) : F] = 2.
Part a holds with K = F ╱γ2、.
a ÷ b. By tower [K: F] = [E : K] = 2. The problems from the previous HW about the splitting field of quadratic equations over number fields give
K = F (α), α F, α2 e F.
(Namely, α = ^∆, where ∆ is the discriminant of the minimal polynomial .æ 0,长 (æ) of any æ 0 ∈ i , æ 0 长 .)
The same argument gives
E = K(β) = F (α, β), β F (α), β2 e F (α).
Assume β2 F ↓
One way to argue:
,·β 2 e F (α)
··β 2 F
· [F (α) : F] = 2
÷ deg曰 β 2 = 2, F (α) = F (β2 ).
But then E = F (α)(β) = F (β2 )(β) = F (β), mβ 2,曰 (x) = x2 + ax + b e F [x], and β is a root of x4 + ax2 + b.
Another way to argue:
Must have deg曰 β = 4. Indeed, the options are
deg曰 β = [F (β) : F] = 2, 4.
If mβ,曰 (x) = x2 + ux + v with u, v e F then u = 0 implies β 2 = _v e F contradicting our assumption on β . On the other hand
÷ β = e F (α),
contradicts the definition of β . One polynomial of degree 4 is easy to find (latin letters for elements of F): (1, α} is a basis of K over F , β 2 e K , therefore
β 2 = a + αb ÷ (β2 _ a)2 = α2 b2 ÷ β 4 _ 2aβ2 + a2 _ α2 b2 = 0. Part b is proved with γ = β and
mβ,曰 (x) = x4 _ 2ax2 + a2 _ α2 b2 e F [x].
〉in4lly! 4ssAme β2 e F ↓
For γ = α(β + 1) have γ e F (α, β), and therefore F (γ) c F (α, β). On the other
hand 2
γ 2 = α2 β 2 + 2α2 β + α2 ÷ β = e F (γ),
and then
α = γ/(β + 1) e F (γ).
Thus
E = F (γ),
and must have deg曰 γ = 4. The polynomial is found by the same steps:
γ 2 = α2 β 2 + 2α2 β + α2 ÷ γ 2 _ α2 (β2 + 1) = 2α2 β,
↘ 」
=a∈曰
then
╱γ2 _ A、2 = 4α2 β 2 ÷ γ4 _ 2Aγ2 + A2 _ B = 0,
↘ 」
=B∈曰
so
m│,曰 (x) = x4 _ 2Ax2 + A2 _ B e F [x].
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2. Let f (x) e E[x], f (x) = xn + an — 1 xn — 1 + . . . + a0 be monic, separable, which splits over E . Let φ e Aut(E) fix the set of roots, f (α) = 0 ÷ f (φ(α)) = 0. Prove that φ(ak ) = ak for all k .
Solution. Let α e E be a root of f . It is given that φ(α) is a root of f as well. At the same time since φ is homo
f(α) = 0 ÷ φ(f(α)) = 0
÷ ╱φ(α)、n + φ(an — 1 ) ╱φ(α)、n — 1 + . . . + φ(a1 ) ╱φ(α)、+ φ(a0 ) = 0. Hence φ(α) is a root of
g(x) = xn + φ(an — 1 )xn — 1 + . . . + φ(a1 ).
The problem asks to prove f(x) = g(x).
In fact, let α 1 , . . . , αn be distinct roots of f . Then the assumption and the injectivity of φ imply φ(α1 ), . . . , φ(αn ) are distinct roots of f as well. (They are just permuted αi .) We showed that φ(α1 ), . . . , φ(αn ) are roots of g . Thus
f(x) = (x - φ(α1 )) . . . (x - φ(αn )) = g(x).
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3. Let [F (α) : F] = n > 1, let mα,F (x) be separable, and let all roots α 1 = α , α2 , . . ., αn of mα,F (x) belong to F (α). Construct n distinct automorphisms φ 1 , . . . , φn e Aut(F (α)/F) such that φi (α) = αi .
Hint: { 1 , α, . . . , αn二 1 } and {1, α i , . . . , αi(n)二 1 } are two bases in 长 (α) . The requirement on 4i suggests a
formula for it . Prove that the formula indeed gives the fields iso either directly or by quoting a theorem .
Solution 1 . This is immediate consequence of the Extension of Isomor- phisms Theorem which we proved in class, 13.1 Th-m 8, 13.4 Th-m 27.
Take, say, α2 . We have α, α2 two roots of an irreducible polynomial p(x) e F [x], p = mα,F , and
F (α) = F (α2 ).
By the Extension of Isomorphisms Theorem, for the isomorphism
id长
there exists the fields isomorphism φ completing the diagram
F (α) φ F (α2 )
F F
satisfying φ(α) = α2 and φ e Aut(F (α)/F).
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Solution 2. According to the hint define F-linear map of F (α), vector space over F , into itself ψ : F (α) - F (α) by the action on the basis
elements ψ(1) = 1, ψ(α) = αi , ..., ψ(αn — 1 ) = α Equivalently, map any element (vector) in F (α) by
a0 + a1 α + . . . an — 1 αn — 1 ,-- a0 + a1 αi + . . . an — 1 α
where all ak e F . This is an F-linear bijective transformation of vector space F (α) since ψ takes a basis to a basis. Moreover, Vu e F by linearity ψ(u) = ψ(u1) = u.
To prove ψ e Aut(F (α)/F) it is left to establish ψ(βγ) = ψ(β)ψ(γ) Vβ, γ e F (α).
Indeed, recast definition of the basis to discover that for any β there is unique polynomial
gβ (x) = b0 + b1 x + . . . + bn — 1 xn — 1 e F [x], deg g < n - 1,
such that β = gβ (α). Then for the product of polynomials have
gβ gγ = gβγ
because
βγ = gβ (α)gγ (α) = ╱gβ (x)gγ (x)、│x=α .
The definition of ψ is
ψ(β) = gβ (αi ).
Then
ψ(β)ψ(γ) = gβ (αi )gγ (αi ) = (gβ gγ )(αi ) = ψ(βγ).
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4. For any odd m find Aut ╱Q╱ 2、/Q、.
Find Aut ╱Q╱l42、/Q、and Aut ╱Q╱l42、/Q╱l2、、. Which of the two ex-
tensions is Galois?
Let S be the splitting field of x4 - 2. Find |Aut(S/Q)| the order of the group.
Solution.
(a) We write E = Q╱ 2、. Take any φ e Aut (E/Q). Since f (x) = xm - 2 is a polynomial over Q we have
f (α) = 0, α e E ÷ φ(α) e E, f (φ(α)) = 0.
For odd m look at the set { 2} c 仑 to find out
兮
k = 1, . . . , m
兮 k = m, α = 2
Thus our φ e Aut(E/Q) satisfies φ(1) = 1, φ( 2) = 2. Elements
1, 2, ╱ 2、2 , . . . , ╱ 2、m — 1
form a basis of vector space E over Q, because f(x) is irreducible over Q by Eisenstein, f(x) = m .^2 ,Q (x). Since φ is a homo of E
φ ╱ ╱ 2、k 、= ╱φ ╱ 2、、k = ╱ 2、k .
Hence every basis vector is fixed by φ, so φ = idE ,
Aut(E/Q) = {idE } |
(b) We write E = Q╱l42、. A basis of E over Q is
1, l42, ╱ l42、2 , ╱ l42、3
because f(x) = x4 - 2 is irreducible over Q by Eisenstein. Take any φ e Aut (E/Q). Let us find its action on the basis vectors.
We have f(α) = 0, α e E ÷ φ(α) e E , f(φ(α)) = 0. All roots of f in 仑 are 士l42, 士i l42. All roots in E are 士l42. Hence the options are
φ( l42) = l42 or φ( l42) = - l42.
In the first case for any other basis vector
φ ╱ ╱l42、k 、= ╱ l42、k ÷ φ = idE .
In the second case we know that φ is a uniquely determined Q-linear bijective transformation of E whose action on the basis is
φ ╱ ╱l42、k 、= (-1)k ╱ l42、k , k = 0, 1, 2, 3.
Does it preserve the multiplication in E? If "yes" then order of the group is 2 and it must be isomorphic to 勿2 , if "no" then the group is trivial.
Apply the (argument from the proof of) previous problem to conclude that there exists ψ e Aut(E/Q) such that ψ( l42) = - l42. This condition uniquely determine the action of ψ on the basis of E which is the same as the action of φ . Hence φ = ψ an automorphism.
Aut(E/Q) 勿2 |
Since |Aut(E/Q)| = 2 4 = [E : Q], the extension is not Galois.
(c) We write E = Q╱^42、, K = Q(^2). The problem asks to find Aut(E/K).
We have fields extensions Q c K c E and [K : Q] = 2 since x2 - 2 is irreducible over Q. By tower must have [E : K] = 2. Trivially Aut(E/K) < Aut(E/Q). The latter group we computed earlier. Hence the options for an element of Aut(E/K) are idE and φ , φ( ^42) = - ^42. The first isomorphism fixes K . The second fixes K as well since
φ(^2) = φ ╱ ^42 . ^42、= ╱ - ^42、╱ - ^42、= ^2.
Thus
Aut(E/K) 勿2 .
Since |Aut(E/K)| = 2 = [E : K], the extension is Galois.
(d) Polynom x4 - 2 is separable. Hence S/Q is a Galois extension. We know Aut(S/Q) G < S4 , |S4 | = 24, and by Galois |Aut(S/Q)| = [S : Q]. Since S = Q(士^2, 士i^2) = Q(i, ^2) compute [S : Q] = 2 . 4 = 8.
Aut(S/Q) |
= 8 |
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2023-06-09