Symmetric Polynomials and the Fundamental Theorem of Symmetric Functions
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1 · Prerequisites
Abstract Algebra
- Binary Operations, Monoids, Groups and Subgroups
- Cosets, Index and Lagrange's Theorem
- Divisibility, Euclidean Domains, Principal Ideal Domains and Unique Factorisation
- Ideals, Quotient Rings and the Isomorphism Theorems for Rings
- Normal Subgroups and Quotient Groups
- Polynomial Rings, the Division Algorithm and Roots
- Rings, Subrings, Integral Domains and Fields
- Simple Field Extensions and the Construction of the Complex Numbers
- Splitting Fields
Real Analysis
2 · Logical flowchart
3 · Items
Definition
Symmetric polynomials as the invariants of variable permutations
Proposition
The symmetric polynomials form a subring
Definition
The elementary symmetric polynomials e₀,e₁,…,eₙ
Theorem
Vieta expansion: ∏ᵢ₌₁ⁿ(t-xᵢ)=∑ₖ₌₀ⁿ(-1)ᵏ eₖ tⁿ⁻ᵏ
Corollary
Vieta's formulas identify the coefficients of a split monic polynomial with elementary symmetric functions of its roots
Definition
Monomial symmetric polynomials indexed by partitions
Theorem
Monomial symmetric polynomials form an R-basis of the symmetric-polynomial ring
Definition
Lexicographic order on exponent tuples and the multidegree of a nonzero polynomial
Lemma
The leading multidegree of a symmetric polynomial is weakly decreasing
Lemma
The leading multidegree of e₁^b₁⋯ eₙ^bₙ is (b₁+⋯+bₙ,b₂+⋯+bₙ,…,bₙ) with coefficient one
Theorem
Every symmetric polynomial over a commutative ring is a polynomial in the elementary symmetric polynomials
Theorem
The elementary symmetric polynomials are algebraically independent over the coefficient ring
Theorem
Fundamental theorem of symmetric polynomials: unique expression as a polynomial in e₁,…,eₙ
Corollary
A symmetric polynomial in the roots of a monic polynomial is a polynomial in its coefficients and lies in the base ring
Definition
Power sums pₖ and complete homogeneous symmetric polynomials hₖ
Proposition
The generating-series identity E(-t)H(t)=1
Corollary
The complete homogeneous symmetric polynomials h₁,…,hₙ freely generate the symmetric-polynomial ring
Theorem
Newton's identities: k eₖ=∑ᵢ₌₁ᵏ(-1)ⁱ⁻¹eₖ₋ᵢpᵢ
Corollary
If n! is invertible, then p₁,…,pₙ freely generate the symmetric-polynomial ring
Definition
The Vandermonde polynomial Δₙ=∏_i<j(xᵢ-xⱼ)
Proposition
The square of the Vandermonde polynomial is symmetric
Definition
The discriminant of a monic polynomial as the coefficient expression of Δₙ²
Theorem
The discriminant is ∏_i<j(αᵢ-αⱼ)² and vanishes exactly when a monic polynomial has a repeated root
Definition
The monic resultant Res(f,g) from the symmetric coefficient expression of ∏ᵢ g(xᵢ)
Theorem
For monic f, Res(f,g)=∏ᵢ g(αᵢ) and it vanishes exactly when f and g have a common root
Theorem
For monic f,g of degrees n,m splitting in a common extension, Res(f,g)=∏ᵢ₌₁ⁿ∏ⱼ₌₁ᵐ(αᵢ-βⱼ)
Corollary
For monic f,g of degrees n,m, Res(f,g)=(-1)ᵐⁿRes(g,f)
Corollary
For monic f, if g-g₁=qf, then Res(f,g)=Res(f,g₁)
Corollary
For monic f of degree n, Res(f,f')=(-1)^n(n-1)/2Disc(f)
Sources
- K. Conrad, Symmetric Polynomials, Sections 1-5
- D. Grinberg, An Introduction to Algebraic Combinatorics, Chapter 7, Sections 7.1-7.2
- D. Grinberg, An Introduction to Algebraic Combinatorics, Chapter 7, Section 7.1
- K. Conrad, Symmetric Polynomials, Section 1
- D. Grinberg, An Introduction to Algebraic Combinatorics, Theorem 7.2.7
- K. Conrad, Symmetric Polynomials, Section 2
- D. Grinberg, An Introduction to Algebraic Combinatorics, Theorem 7.1.16
- J. S. Milne, Fields and Galois Theory, Theorem 5.36 and Remark 5.37
- K. Conrad, Symmetric Polynomials, Sections 1-2
- K. Conrad, Symmetric Polynomials, Section 3
- J. S. Milne, Fields and Galois Theory, Proposition 4.35 through Example 4.37
- J. S. Milne, Fields and Galois Theory, Proposition 4.35
- J. S. Milne, Fields and Galois Theory, definition preceding Proposition 4.35
- J. S. Milne, Fields and Galois Theory, Proposition 4.35(a)
- J. S. Milne, Fields and Galois Theory, Proposition 4.35(c)
- J. S. Milne, Fields and Galois Theory, Proposition 4.35 and discriminant discussion