Splitting fields are crucial in understanding polynomial roots and field extensions. They're the smallest fields containing all roots of a given polynomial, . This concept bridges basic field theory with more advanced Galois theory.
Constructing splitting fields involves to the base field step-by-step. The process reveals important properties like field degree and structure, connecting polynomial behavior to abstract algebra and number theory.
Splitting Fields for Polynomials
Definition and Properties
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A of a polynomial f(x) over a field F is the smallest of F that contains all the roots of f(x)
The splitting field is obtained by adjoining all the roots of f(x) to the base field F (adjoining means adding elements to the field to create a larger field)
The splitting field is unique up to isomorphism, meaning that any two splitting fields of a polynomial over the same base field are isomorphic (have the same structure and properties)
The degree of the splitting field over the base field is equal to the order of the Galois group of the polynomial (the Galois group is the group of field automorphisms that fix the base field)
Examples
The splitting field of x2−2 over Q is Q(2), obtained by adjoining 2 to Q
The splitting field of x3−2 over Q is Q(32,ω), where ω is a primitive cubic of unity
Constructing Splitting Fields
Step-by-Step Process
To construct a splitting field, first factor the polynomial completely over the base field
If all the roots are already in the base field, then the base field itself is the splitting field
If some roots are not in the base field, adjoin one root at a time to the base field until all roots are included. The resulting field is the splitting field
The process of adjoining a root α to a field F is denoted by F(α) and is the smallest field containing both F and α
When adjoining multiple roots, the order in which they are adjoined does not matter; the resulting splitting field will be the same up to isomorphism
Examples
To construct the splitting field of x4−1 over Q:
Factor: x4−1=(x−1)(x+1)(x−i)(x+i)
Adjoin i to Q: Q(i)
All roots are in Q(i), so this is the splitting field
To construct the splitting field of x3−2 over Q:
Adjoin 32 to Q: Q(32)
Adjoin ω (a primitive cubic root of unity) to Q(32): Q(32,ω)
All roots are in Q(32,ω), so this is the splitting field
Uniqueness of Splitting Fields
Proving Uniqueness up to Isomorphism
Let E and E′ be two splitting fields of a polynomial f(x) over a field F. To prove uniqueness, we need to show that E and E′ are isomorphic
Define a homomorphism ϕ:F[x]/(f(x))→E by sending x to a root α of f(x) in E. This homomorphism is surjective because E is generated by α over F
Similarly, define another homomorphism ϕ′:F[x]/(f(x))→E′ by sending x to a root α′ of f(x) in E′. This homomorphism is also surjective
By the first isomorphism theorem, F[x]/(f(x))≅E and F[x]/(f(x))≅E′. Therefore, E≅E′, proving the uniqueness of splitting fields up to isomorphism
Examples
The splitting field of x2−2 over Q is unique up to isomorphism, whether constructed as Q(2) or Q(−2)
The splitting field of x4−2 over Q is unique up to isomorphism, whether constructed as Q(42,i) or Q(42,−i)
Degree of Splitting Fields
Determining the Degree
The degree of a splitting field E over its base field F is equal to the order of the Galois group G of the polynomial f(x) over F, denoted as [E:F]=∣G∣
The Galois group G is the group of all automorphisms of E that fix F elementwise
To find the degree of the splitting field, determine the order of the Galois group by examining the permutations of the roots of f(x) that preserve the field operations
If the polynomial f(x) factors into linear factors over F, then the degree of the splitting field is equal to the degree of the polynomial
If f(x) is irreducible over F and has degree n, then the degree of the splitting field is a divisor of n!
Examples
The splitting field of x2−2 over Q has degree 2 over Q because the Galois group has order 2 (the identity and the that swaps 2 and −2)
The splitting field of x4−2 over Q has degree 8 over Q because the Galois group has order 8 (the dihedral group D4)