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Thursday, October 31, 2013

Quaternion Galois Group (14.2.27)

Dummit and Foote Abstract Algebra, section 14.2, exercise 27:

MathJax TeX Test Page Let α=\sqrt{(2+\sqrt{2})(3+\sqrt{3})} and consider the extension E=\mathbb{Q}(α).
(a) Show that a=(2+\sqrt{2})(3+\sqrt{3}) is not a square in F=\mathbb{Q}(\sqrt{2},\sqrt{3}).
(b) Conclude from (a) that [E~:~\mathbb{Q}]=8. Prove that the roots of the minimal polynomial over \mathbb{Q} for α are the 8 elements \pm \sqrt{(2 \pm \sqrt{2})(3 \pm \sqrt{3})}
...
(f) Conclude \text{Gal}(E/\mathbb{Q})≅Q_8.

Proof: (a) We observe that if a=c^2 then aφa=c^2φc^2=(cφc)^2 where φ is the automorphism of F fixing \sqrt{2} and negating \sqrt{3}. Since cφc is actually the image of c under N_{F/\mathbb{Q}(\sqrt{2})} we have cφc=\sqrt{aφa}=\sqrt{6(3+\sqrt{3})^2}=\pm (3\sqrt{2}+3\sqrt{6})∈\mathbb{Q}(\sqrt{2}) and now \sqrt{6}∈\mathbb{Q}(\sqrt{2}), a contradiction.

(b-f) (We take a slightly different path from the authors, especially for parts c and d) We have shown [\mathbb{Q}(\sqrt{2},\sqrt{3},α)~:~\mathbb{Q}]=8. Since \dfrac{α^2}{2+\sqrt{2}}-3=\sqrt{3}, we have \mathbb{Q}(\sqrt{2},\sqrt{3},α)=\mathbb{Q}(\sqrt{2},α)=E(\sqrt{2}). Assume [E(\sqrt{2})~:~E]=2; then E(\sqrt{2}) is Galois of degree 2 over E (splitting x^2-2), and the map φ:\sqrt{2}↦-\sqrt{2} is an automorphism of E(\sqrt{2}) fixing E. But φ is in particular an isomorphism of F allowing us to observe φ(α^2)=φ((2+\sqrt{2})(3+\sqrt{3}))=(2-\sqrt{2})(3+\sqrt{3}) \neq α^2, a contradiction. So E=E(\sqrt{2}) and [E~:~\mathbb{Q}]=8.

Now as we saw above, E is the splitting field for x^2-α^2 over F. Therefore every automorphism of F extends to an automorphism of E. This is an order 4 subgroup of \text{Aut}(E/\mathbb{Q}). Since E/F is Galois we also have the automorphism ψ:α↦-α fixing F. Letting n=|\text{Aut}(E/\mathbb{Q})| by Lagrange we see 4~|~n, by Galois we see n≤8, and by counting we see n > 4, so that n=8 and E/\mathbb{Q} is Galois.

We see that the 8 elements mentioned above are distinct by observing squares, and since φ(x^2)=φ(x)^2 for general automorphisms we have φ(x)=\pm \sqrt{φ(x^2)}. Letting H=\text{Aut}(E/F) we see 1,ψ form a set of right coset representatives for H in \text{Aut}(E/\mathbb{Q}). By letting λ∈H fix \sqrt{3} and negate \sqrt{2}, for example, we see λψ(α^2)=(2-\sqrt{2})(3+\sqrt{3}) and thus λψ(α)=\pm \sqrt{(2-\sqrt{2})(3+\sqrt{3})}. Similarly, we can see any automorphism maps α to one of the 8 forms above, and thus must map to all of them, and these are the 8 distinct roots of the minimal polynomial for α over \mathbb{Q}.

Let σ map α to β=\sqrt{(2-\sqrt{2})(3+\sqrt{3})}. We see σ(α^2)=β^2 so that σ(\sqrt{2})=-\sqrt{2} and σ(\sqrt{3})=\sqrt{3}, and together with αβ=\sqrt{2}(3+\sqrt{3}) we have σ(αβ)=-αβ and thus σ(β)=-α. Now σ can be seen to be of order 4 and together with \tau mapping α to γ=\sqrt{(2+\sqrt{2})(3-\sqrt{3})} we similarly find the relations σ^4=\tau^4=1, σ^2=\tau^2, and σ\tau = \tau σ^3 (keeping in mind β=\dfrac{\sqrt{2}(3+\sqrt{3})}{α}), so that \text{Gal}(E/\mathbb{Q})≅Q_8.~\square

Saturday, October 26, 2013

Calculation of Galois Groups (14.2.10,12)

Dummit and Foote Abstract Algebra, section 14.2, exercises 10, 12:

MathJax TeX Test Page 10. Determine the Galois group of the splitting field over \mathbb{Q} of x^8-3.
12. Determine the Galois group of the splitting field over \mathbb{Q} of x^4-14x^2+9.

Proof: (10) Letting θ=\zeta_8=\dfrac{\sqrt{2}}{2}+\dfrac{\sqrt{2}}{2}i, we have the 8 roots of this polynomial are θ^a\sqrt[8]{3} for a=0,1,...,7. Therefore the splitting field for this polynomial is \mathbb{Q}(\sqrt[8]{3})(\sqrt{2})(i). We note x^8-3 is irreducible over \mathbb{Q} by Eisenstein, so the first extension is degree 8, and assuming x^2-2 isn't irreducible over \mathbb{Q}(\sqrt[8]{3}) leads to a solution(a_0+a_1\sqrt[8]{3}+...+a_7\sqrt[8]{3}^7)^2=2However, we notice the coefficient of the basis element 1 here isa_0^2+6a_1a_7+6a_2a_6+6a_3a_5+3a_4^2=2We notice that the integral domain of elements of the form b_0+b_1\sqrt[8]{3}+...+b_7\sqrt[8]{3}^7 for integers b_i has for field of fractions \mathbb{Q}(\sqrt[8]{3}), because the latter contains the former and the former contains the latter by writing fractional coefficients under a common denominator. Thus by Gauss's lemma we may assume the a_i are integers, and reducing modulo 3 the equality is impossible. Therefore \mathbb{Q}(\sqrt[8]{3})(\sqrt{2}) is of degree 16, and since this field is contained in \mathbb{R} we see K=\mathbb{Q}(\sqrt[8]{3})(\sqrt{2})(i) is of degree 32 over \mathbb{Q}.

We see there are 8*2*2=32 permutations of the roots, therefore these are all automorphisms, so \text{Gal}(K/\mathbb{Q}) is a group generated by the automorphismsα~:~\sqrt[8]{3}↦θ\sqrt[8]{3}β~:~\sqrt{2}↦-\sqrt{2}γ~:~i↦-iWe see these elements satisfy α^8=β^2=γ^2=1, βγ=γβ, βα=α^5β, and γα=α^3γ, and also these relations on a free group of three generators is sufficient to write any element in the form α^aβ^bγ^c, of which there are 32 combinations, so this is precisely the set of relations.\text{Gal}(K/\mathbb{Q})=<α,β,γ~|~α^8=β^2=γ^2=1,~βγ=γβ,~βα=α^5β,~γα=α^3γ>Now, since 3^2≡5^2≡7^2≡1~\text{mod }8 we observe \text{Aut}(Z_8)=Z_2^2, so letting φ be the isomorphism between these two groups, letting a generate Z_8, and b,c∈Z_2^2 be such that φ(b)(a)=a^5 and φ(c)(a)=a^3, we observe the same relations between these elements in Z_2^2 \rtimes_φ Z_8 also of order 32, so that finally we may say\text{Gal}(K/\mathbb{Q})=Z_2^2 \rtimes_φ Z_8 (12) Finding the roots of the polynomial in x^2, we obtain the solutions α=\sqrt{7+2\sqrt{10}}, β=\sqrt{7-2\sqrt{10}}, , and . We note αβ=\sqrt{7^2-(2\sqrt{10})^2}=3, so that β=3/α and the splitting field is merely \mathbb{Q}(α). We see that \mathbb{Q}(α) is of degree 4 since the polynomial points to ≤4 and \mathbb{Q}(\sqrt{10}) ⊂ \mathbb{Q}(α) (for the proper inclusion, consider a solution to x^2-α^2 over \mathbb{Z}(\sqrt{10}) by Gauss's lemma). The automorphisms of this extension must be the permutations of α about the roots of its minimal polynomial, so we observe φ~:~α ↦ -α, ψ~:~α↦β=3/α, and φψ are all of order 2, so\text{Gal}(\mathbb{Q}(α)/\mathbb{Q})=Z_2^2~\square

Friday, October 25, 2013

Computations with Galois Theory (14.2.1-3)

Dummit and Foote Abstract Algebra, section , exercise :

MathJax TeX Test Page 1. Determine the minimal polynomial over \mathbb{Q} for the element \sqrt{2}+\sqrt{5}.
2. Determine the minimal polynomial over \mathbb{Q} for the element 1+\sqrt[3]{2}+\sqrt[3]{4}.
3. Determine the Galois group of (x^2-2)(x^2-3)(x^2-5).

Proof: Lemma: Let K/F be Galois over a perfect field and let α∈K. Then the minimal polynomial of α over F is the squarefree part of the polynomial\prod_{σ∈\text{Gal}(K/F)}(x-σα)Proof: We show \{σα~|~σ∈\text{Gal}(K/F)\} is the full set of zeros for the minimal polynomial p(x). They are all zeros as the automorphisms fix the coefficients of p(x), and as well for any other root β of p(x) we have the isomorphism F(α)→F(β) and extending automorphism K preserving this map, since being Galois K is a mutual splitting field for F, F(α), and F(β).

As well, F being perfect, p(x) is separable and thus has no repeated roots, and we may now say p(x)~|~\prod_{σ∈\text{Gal}(K/F)}(x-σα). Since the zeros of the latter are precisely the (nonrepeated) zeros of the former, we have the squarefree part is indeed p(x).~\square This solves the problem of determining minimal polynomials (over perfect fields) when the Galois group of a containment Galois extension is known (and still provides much information when F isn't perfect).

(1) We see \sqrt{2}+\sqrt{5}∈\mathbb{Q}(\sqrt{2},\sqrt{5})=\mathbb{Q}(\sqrt{2})(\sqrt{5}), where the latter is computed to be Galois (splitting field of (x^2-2)(x^2-5)) of degree 4. The four automorphisms must be the four uniquely defined by the identity, α~:~\sqrt{2}↦-\sqrt{2}, β~:~\sqrt{5}↦-\sqrt{5}, and the composite αβ. Thus the four roots of the minimal polynomial are \pm \sqrt{2} \pm \sqrt{5} and the minimal polynomial is calculated to be x^4-14x^2+9.

(2) We put the element in the Galois extension \mathbb{Q}(ρ,\sqrt[3]{2}) and observe the effect of the elements of the Galois group previously determined. The result is the polynomial(x-1-\sqrt[3]{2}-\sqrt[3]{4})(x-1-ρ\sqrt[3]{2}-ρ^2\sqrt[3]{4})(x-1-ρ\sqrt[3]{4}-ρ^2\sqrt[3]{2})=x^3-3x^2-3x-1 (3) With some simple algebra we may see x^2-5 is irreducible over the field \mathbb{Q}(\sqrt{2},\sqrt{3}) of degree 4 over \mathbb{Q}, so the splitting field in question \mathbb{Q}(\sqrt{2},\sqrt{3},\sqrt{5}) is Galois of degree 8. We see the automorphisms are precisely the automorphisms generated by the maps α~:~\sqrt{2}↦-\sqrt{2}, β~:~\sqrt{3}↦-\sqrt{3}, γ~:~\sqrt{5}↦-\sqrt{5}. By noting that these automorphisms all commute and are all of order 2, we conclude the Galois group is Z_2^3.~\square

Sunday, October 20, 2013

Automorphisms of Polynomial Rings over Fields (14.1.6, 8-9)

Dummit and Foote Abstract Algebra, section 14.1, exercises 6, 8-9:

MathJax TeX Test Page 6. Let k be a field. Show that the automorphisms of k[t] fixing k are precisely the linear transformations defined by t ↦ at+b for a \neq 0.
8. Show that the automorphism of k(t) fixing k are precisely the fractional linear transformations defined by t ↦ \dfrac{at+b}{ct+d} for ad-bc \neq 0.
9. Determine the fixed field of the automorphism t ↦ t+1 of k(t).

Proof: (6) Let φ be such a mapping. It is seen to be a ring homomorphism as evaluation at any polynomial is seen to be a ring homomorphism. Moreover, letting k[x]_i denote the subspace spanned by 1,t,...,t^i over k, when a \neq 0 we see the basis elements of k[x]_i are mapped to linear combinations of its preimage basis, showing φ is bijective on k[x]_i and by extension on k[x] since ∪k[x]_i=k[x].

Conversely, any automorphism fixing k is uniquely defined by its action on t, so observe the polynomial f(t)=φ(t). We have φ(g(t))=g(f(t))=t for some g(t)∈k[x] by surjectivity, and since \text{deg }g(f(t)) = \text{deg }f(t)~\text{deg }g(t), necessarily f(t)=at+b for some a \neq 0.

(8) As before, evaluation is an endomorphism. When ad-bc \neq 0, at+b and ct+d are relatively prime (one of them may be in k) and by 13.2.18 [k(t)~:~\text{img }φ]=1 so φ is surjective. Now, assume f(\dfrac{at+b}{ct+d})=0 for f(t) = ∑a_kt^k (f(t) being fractional implies the existence of a nonfractional polynomial satisfying such). Letting n=\text{deg }f(t) and observing (ct+d)^nf(\dfrac{at+b}{ct+d})= a_n(at+b)^n + (ct+d)g(t) = 0 for some polynomial g(t), we see by the fact that k[t] is a UFD that ct+d is a unit and injectivity follows by (6).

As before, all endomorphisms are evaluations, and writing them in the form φ~:~t↦p(t)/q(t) for relatively prime p(t),q(t) by 13.2.18 for it to be surjective necessarily the greatest degree is 1. Clearly ad-bc \neq 0 as otherwise \text{img }φ=k.

(9) By the previous this is indeed an automorphism. Now, let f(t)=\dfrac{pt)}{q(t)} for relatively prime p(t),q(t) and monic p(t) be a typical element of the fixed field, i.e. f(t)=f(t+1). Then \dfrac{p(t)}{q(t)}=\dfrac{p(t+1)}{q(t+1)} and p(t)q(t+1)=p(t+1)q(t). Assuming p(t) \neq p(t+1) implies p(t) \not \mid p(t+1) since they are monic of the same degree, so there is some irreducible factor on the left not present on the right, a contradiction. Now p(t)q(t+1)=p(t)q(t) so q(t)=q(t+1) and it suffices to find the collection of polynomials in k[t] fixed by t ↦ t+1.

If k is of characteristic 0 then f(t)=f(t+1) implies f(α)=0 implies f(α+1)=0, so f(t) has no zeros in any field and f(t)∈k. In this case the fixed field is merely k. Now consider the polynomial λ(t)=t(t-1)...(t-(p-1)) in k[t], where p is the characteristic of k. Clearly λ(t)=λ(t+1), so all polynomials generated as a ring by λ(t) and k are fixed by t↦t+1. Conversely, if f(t)=f(t+1) and f(0)=a_0, then for the polynomial F(t)=f(t)-f(0) we have F(t)=F(t+1) and F(0)=0 so also F(1),...,F(p-1)=0 and λ(t)~|~F(t). By induction on degree F(t)/λ(t) is in the ring generated by λ(t) and k and so too is f(t), so this ring in k[t] (also known as the image of φ~:~f(t)↦f(λ(t)) on k[t], to provide a way of efficiently determining whether a polynomial is fixed by t↦t+1) extended to a field in k(t) is precisely the field fixed by t↦t+1.~\square

Friday, October 18, 2013

Canonical forms of the Fröbenius Endomorphism (13.6.11-12)

Dummit and Foote Abstract Algebra, section 13.6, exercises 11-12:

MathJax TeX Test Page 11-12. Let φ denote the Fröbenius map φ:\mathbb{F}_{p^n}→\mathbb{F}_{p^n}x↦x^pFind the rational and Jordan (when it exists) canonical form of φ.

Proof: (11) We saw φ^n=1, so since [\mathbb{F}_{p^n}~:~\mathbb{F}_p]=n we note φ is an n \times n matrix and the invariant factors of φ all divide x^n-1. Assume there are m > 1 invariant factors, so h_1(x)~|~h_2(x) are invariant factors and write h_2(x)=f(x)h_1(x). Observe elements of the form \bigoplus_{k=1}^m a_k where f(x)~|~a_k when k=2, and a_k=0 when k > 2. These are all in the kernel of h_1(x), and yet there are more than p^a of them, implying there are more than p^a solutions to the polynomial of degree p^a represented by the linear transformation h_1(x), contradiction. Therefore the sole invariant factor is one of degree n dividing x^n-1, necessarily x^n-1 itself and the Jordan canonical form is the matrix with 1s along the subdiagonal and a 1 in 1,n.

(12) Let n=p^km for p \not | m. We see x^n-1=(x^m-1)^{p^k} in \mathbb{F}_{p^n}[x], so that by previous investigations the invariant factors are some powers of (x-\alpha) where \alpha is a power of an n^{th} primitive root of unity, and moreover there are m such distinct roots. By the same reason as above, and since the degree of the product of the polynomials must be n, the Jordan canonical form (when it exists, i.e. when there is an m^{th} primitive root in \mathbb{F}_{p^n}, iff m~|~p^n-1) of φ is the matrix with Jordan blocks (x-\zeta_{m})^{p^k}.

Friday, October 11, 2013

Finite Extensions of Q and Roots of Unity (13.6.5)

Dummit and Foote Abstract Algebra, section 13.6, exercise 5:

MathJax TeX Test Page Prove there are only a finite number of roots of unity in any finite extension K of \mathbb{Q}.

Proof: Let ψ(k)=p_1p_2...p_k, where p_i is the i^{th} prime.

Lemma 1 (Local Extrema of Totient Ratio): If n≤ψ(k) then φ(n)/n ≥ \dfrac{(p_1-1)(p_2-1)...(p_k-1)}{p_1p_2...p_k}. Proof: Collect n such that φ(n)/n is minimal, and then choose n=q_1^{α_1}q_2^{α_2}...q_m^{α_m} minimal from this collection. Assume α_i > 1 for some i; thenφ(n)/n=\dfrac{q_1^{α_1-1}(q_1-1)q_2^{α_2-1}(q_2-1)...q_m^{α_m-1}(q_m-1)}{q_1^{α_1}q_2^{α_2}...q_m^{α_m}}=\dfrac{(q_1-1)(q_2-1)...(q_m-1)}{q_1q_2...q_m}=φ(n/q_i)/(n/q_i)and n/q_i < n, violating minimality. So n is squarefree. Let p_j be the smallest prime not dividing n, which must be ≤p_k else n=ψ(k) and φ(n)/n equals the bound given above. If there is no prime larger than p_k dividing n then set m=p_jn ≤ ψ(k), and otherwise let q_v be this prime and set m=p_jn/q_v < n ≤ ψ(k). In the first case we see φ(m)/m=\dfrac{p_j-1}{p_j}φ(n)/n < φ(n)/n and in the second φ(m)/m = \dfrac{q_v(p_j-1)}{(q_v-1)p_j}φ(n)/n < φ(n)/n, invariably violating minimality. Thus the bound holds.\square

Lemma 2: φ(n)→∞. Proof: Choose finite positive z, and let k be such that (p_1-1)(p_3-1)...(p_k-1) > z (index 2 is missing). We show when n > ψ(k) that φ(n) > z. Let k' be such that ψ(k'-1) < n ≤ ψ(k') so that k' > k ≥ 3. We observeφ(n) = (φ(n)/n)n ≥ \dfrac{(p_1-1)(p_2-1)...(p_{k'}-1)}{p_1p_2...p_{k'}}p_1p_2...p_{k'-1} =\dfrac{(p_1-1)(p_2-1)...(p_{k'}-1)}{p_{k'}} ≥ (p_1-1)(p_3-1)...(p_{k'-1}-1) ≥(p_1-1)(p_3-1)...(p_k-1) > z~~\squareNow, since there are only a finite number of primitive roots for any n, K must contain n^{th} primitive roots for n arbitrarily large. Since the degree φ(n) of the cyclotomic minimal polynomial for these primitive roots also becomes arbitrarily large, we must have K/\mathbb{Q} is not finite.~\square

Facts About General Roots of Unity (13.6.1-4)

Dummit and Foote Abstract Algebra, section 13.6, exercises 1-4:

MathJax TeX Test Page 1. Suppose m and n are relatively prime integers. Prove \zeta_m \zeta_n is a primitive mn^{th} root of unity.

2. Let d~|~n. Prove \zeta_n^d is a primitive (n/d)^{th} root of unity.

3. Prove that if a field contains the n^{th} roots of unity for odd n then it also contains the 2n^{th} roots of unity.

4. Prove that if n=p^km for p~\not \mid~m then there are precisely m distinct n^{th} roots of unity over a field of characteristic p.

Proof: (1) Let the field in question be of characteristic p or 0, and assume without loss that p~\not \mid~n. Further assume that p~\not \mid m or that the characteristic is 0. Then by (4) we may assume \zeta_m,\zeta_n are of orders m,n respectively. We see (\zeta_m\zeta_n)^{mn}=1 so the order of \zeta_m\zeta_n is ≤mn. As well, (\zeta_m\zeta_n)^m=\zeta_n^m is of order n, so n divides the order of \zeta_m\zeta_n. Similarly m divides this order and \zeta_m\zeta_n is of order mn and thus a primitive mn^{th} root of unity (still keeping mind of the case of characteristic p, as the mn zeros of x^{mn}-1 are all distinct).

Now assume exclusively characteristic p and m=p^km' for p~\not \mid~m'. Then x^m-1=(x^{m'}-1)^{p^k} and so \zeta_m is of order m' and \zeta_m\zeta_n is of order m'n. Finally, we see x^{mn}-1=(x^{m'n}-1)^{p^k} so there are exactly m'n distinct solutions and thus \zeta_m\zeta_n is a primitive mn^{th} root.

(2) Suppose characteristic 0. Then clearly \zeta_n^d is of order ≤n/d and at least of this order, so is a primitive root. Suppose characteristic p and write d=p^jd' and n=p^kn' for maximal j,k. Then x^{n/d}-1=(x^{n'/d'}-1)^{p^{k-j}} has n'/d' solutions, so since \zeta_n is of order n' by (4) and thus \zeta_n^d of order n'/d', we thus have \zeta_n^d is primitive.

(3) Since -1 is the 2^{nd} root of unity, by (1) -\zeta_n are the 2n^{th} roots of unity.

(4) We have x^n-1=(x^m-1)^{p^k} where D_x x^m-1 = mx \neq 0 has no zeros in common with x^m-1, so there are m distinct roots of x^m-1 and thus of x^n-1.\square

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~|~n⇔x^d-1~|~x^n-1 in \mathbb{Z}[x].

4. Let 1 < a ∈ \mathbb{Z}. Prove for positive n,d∈\mathbb{Z} that d~|~n⇔a^d-1~|~a^n-1. Conclude in particular that \mathbb{F}_{p^d}⊆\mathbb{F}_{p^n}⇔d~|~n.

Proof: (3) () Evidently (x^d-1)(x^{n-d}+x^{n-2d}+...+x^d+1)=x^n-1. () Write f(x)(x^d-1)=x^n-1 and assume d~\not \mid~n so n=qd+r for some 0 < r < d. We thus have every solution of x^d-1 is a solution of x^n-1 in any field containing \mathbb{Z}. Let \zeta be a primitive d^{th} root of unity. Then \zeta^n-1=\zeta^{qd+r}-1=\zeta^r-1 \neq 0, a contradiction.

(4a) First we note that a^n-1 ≡ a^r-1~\text{mod }a^d-1 if n≡r~\text{mod }d. This is because, after writing n=qd+r for 0 ≤ r < d, we have 1≡(a^d)^q ≡ a^{qd} ≡ a^{n-r} and now a^n ≡ a^r. Since 0 ≤ a^r - 1 < a^d - 1 we have a^n - 1 ≡ 0 if and only if n ≡ 0~\text{mod }d.

(4b) Assume d~|~n; then p^d-1~|~p^n-1 so x^{p^d-1}-1~|~x^{p^n-1}-1 and the splitting field of the latter contains the former, i.e. \mathbb{F}_{p^d}⊆\mathbb{F}_{p^n}. Assume \mathbb{F}_{p^d}⊆\mathbb{F}_{p^n}; then we see these are exactly the splitting fields of x^{p^d-1}-1 and x^{p^n-1}-1 respectively, andx^{p^d-1}-1=\prod_{α∈\mathbb{F}_{p^d}} (x-α)~|~\prod_{α∈\mathbb{F}_{p^n}} (x-α) = x^{p^n-1}-1implying p^d-1~|~p^n-1 implying d~|~n.~\square

Sunday, October 6, 2013

Splitting Field Computations (13.4.1-4)

Dummit and Foote Abstract Algebra, section 13.4, exercises 1-14:

MathJax TeX Test Page Determine the degree of the splitting field over \mathbb{Q} for the following polynomials:
x^4-2
x^4+2
x^4+x^2+1
x^6-4

Proof: x^4-2: Letting α be a solution to this polynomial, we see the elements α,,, and -iα are the four distinct solutions. Therefore we have the splitting field contains α and i and also a field containing α and i contains the splitting field, so the splitting field is precisely \mathbb{Q}(α,i). We shall show x^4-2 is irreducible over \mathbb{Q}[x] by first observing it has no roots in \mathbb{Z} and also does not decompose into two quadratics:x^4-2=(x^2+ax+b)(x^2+cx+d)bd=-2⇒d=-2/bad+bc=0⇒c=2a/b^2b+d+ac=0⇒a^2=-b(b^2-2)/2a+c=0⇒a=2a/b^2⇒a=0,b^2-2=0⇒b \not ∈ \mathbb{Z}Thus [\mathbb{Q}(α)~:~\mathbb{Q}]=4, and since \mathbb{Q}(α)⊂\mathbb{R} we have x^2+1 irreducible over \mathbb{Q}(α), so the computed degree is 8.

x^4+2: Again letting α be a root we have the splitting field is \mathbb{Q}(α,i). We shall show x^4+2 is irreducible over \mathbb{Q}(i) by observing it doesn't have a root (else the splitting field would be \mathbb{Q}(i) despite the fact that α^2=\pm \sqrt{2}i \not ∈ \mathbb{Q}(i)) and by computations similar as above it doesn't decompose into quadratics unless there exists b such that b^2-2=0, despite \pm \sqrt{2} \not ∈ \mathbb{Q}(i). Thus the degree is 8.

x^4+x^2+1: After treating this as a quadratic in x^2 and applying some algebra, we come to the factorization in \mathbb{C}[x]x^4+x^2+1=(x-1/2-\sqrt{3}/2i)(x-1/2+\sqrt{3}/2i)(x+1/2-\sqrt{3}/2i)(x+1/2+\sqrt{3}/2i)Therefore the splitting field is precisely \mathbb{Q}(\sqrt{-3}) and the degree is 2.

x^6-4: We observe x^6-4=(x^3-2)(x^3+2). Letting α be a solution to x^3-2 and \zeta being a primitive third root of unity, we see α, \zeta α, and \zeta^2 α are the three solutions. As well, , -\zeta α, and -\zeta^2 α are the solutions to x^3+2. Therefore the splitting field is \mathbb{Q}(α,\zeta). Letting α be the positive real solution, since \zeta is not real and of degree two we must have the degree over \mathbb{Q} is 2*3=6.