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4.4 Elementary and Complete Symmetric Functions

6 min readjuly 30, 2024

Symmetric functions are the building blocks of algebraic combinatorics. They come in two flavors: elementary and complete. These functions form the basis for understanding how polynomials behave when their variables are shuffled around.

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Top images from around the web for Elementary vs Complete Symmetric Functions
Top images from around the web for Elementary vs Complete Symmetric Functions
Top images from around the web for Elementary vs Complete Symmetric Functions

Elementary symmetric functions sum up products of distinct variables, while complete symmetric functions include all possible monomials. Together, they provide powerful tools for analyzing symmetry in polynomials and solving complex algebraic problems.

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Top images from around the web for Elementary vs Complete Symmetric Functions
Top images from around the web for Elementary vs Complete Symmetric Functions
Top images from around the web for Elementary vs Complete Symmetric Functions

Elementary vs Complete Symmetric Functions

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Top images from around the web for Elementary vs Complete Symmetric Functions
Top images from around the web for Elementary vs Complete Symmetric Functions
Top images from around the web for Elementary vs Complete Symmetric Functions

Definition and Properties

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  • Elementary symmetric functions are defined as the sums of all products of k distinct variables for k = 1, 2, ..., n, where n is the total number of variables
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Top images from around the web for Elementary vs Complete Symmetric Functions
 - For example, if the variables are x_1, x_2, x_3, the elementary symmetric functions are:
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Top images from around the web for Elementary vs Complete Symmetric Functions
      - e_1 = x_1 + x_2 + x_3
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Top images from around the web for Elementary vs Complete Symmetric Functions
      - e_2 = x_1x_2 + x_1x_3 + x_2x_3
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Top images from around the web for Elementary vs Complete Symmetric Functions
      - e_3 = x_1x_2x_3
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Top images from around the web for Elementary vs Complete Symmetric Functions
  • Complete symmetric functions are defined as the sums of all monomials of k in n variables for k = 0, 1, 2, ..., where n is the total number of variables
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Top images from around the web for Elementary vs Complete Symmetric Functions
 - For example, if the variables are x_1, x_2, x_3, the complete symmetric functions are:
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Top images from around the web for Elementary vs Complete Symmetric Functions
      - h_0 = 1
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Top images from around the web for Elementary vs Complete Symmetric Functions
      - h_1 = x_1 + x_2 + x_3
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Top images from around the web for Elementary vs Complete Symmetric Functions
      - h_2 = x_1^2 + x_2^2 + x_3^2 + x_1x_2 + x_1x_3 + x_2x_3
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Top images from around the web for Elementary vs Complete Symmetric Functions
      - h_3 = x_1^3 + x_2^3 + x_3^3 + x_1^2x_2 + x_1^2x_3 + x_2^2x_1 + x_2^2x_3 + x_3^2x_1 + x_3^2x_2 + x_1x_2x_3
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Top images from around the web for Elementary vs Complete Symmetric Functions
  • Both elementary and complete symmetric functions are special cases of symmetric functions, which are polynomials invariant under any permutation of the variables
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Top images from around the web for Elementary vs Complete Symmetric Functions
  • The of degree k is denoted as , and the of degree k is denoted as h_k
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Top images from around the web for Elementary vs Complete Symmetric Functions
Top images from around the web for Elementary vs Complete Symmetric Functions
Top images from around the web for Elementary vs Complete Symmetric Functions

Basis for Symmetric Functions

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Top images from around the web for Elementary vs Complete Symmetric Functions
  • The elementary symmetric functions form a basis for the ring of symmetric functions, while the complete symmetric functions form another basis
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Top images from around the web for Elementary vs Complete Symmetric Functions
 - This means that any symmetric function can be uniquely expressed as a polynomial in either the elementary symmetric functions or the complete symmetric functions
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Top images from around the web for Elementary vs Complete Symmetric Functions
 - For example, the power sum symmetric function p_k = x_1^k + x_2^k + ... + x_n^k can be expressed in terms of elementary or complete symmetric functions
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Top images from around the web for Elementary vs Complete Symmetric Functions
Top images from around the web for Elementary vs Complete Symmetric Functions
Top images from around the web for Elementary vs Complete Symmetric Functions

Generating Functions for Symmetric Functions

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Top images from around the web for Elementary vs Complete Symmetric Functions
Top images from around the web for Elementary vs Complete Symmetric Functions
Top images from around the web for Elementary vs Complete Symmetric Functions

Definition and Formulas

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Top images from around the web for Elementary vs Complete Symmetric Functions
  • The generating function for elementary symmetric functions is defined as E(t) = Σ_{k=0}^∞ e_k t^k = ∏_{i=1}^n (1 + x_i t), where x_1, x_2, ..., x_n are the variables and t is a formal variable
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 - The coefficients of the generating function E(t) correspond to the elementary symmetric functions
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Top images from around the web for Elementary vs Complete Symmetric Functions
 - For example, if n = 3, E(t) = (1 + x_1t)(1 + x_2t)(1 + x_3t) = 1 + (x_1 + x_2 + x_3)t + (x_1x_2 + x_1x_3 + x_2x_3)t^2 + x_1x_2x_3t^3
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Top images from around the web for Elementary vs Complete Symmetric Functions
  • The generating function for complete symmetric functions is defined as H(t) = Σ_{k=0}^∞ h_k t^k = ∏_{i=1}^n (1 - x_i t)^(-1), where x_1, x_2, ..., x_n are the variables and t is a formal variable
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Top images from around the web for Elementary vs Complete Symmetric Functions
 - The coefficients of the generating function H(t) correspond to the complete symmetric functions
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Top images from around the web for Elementary vs Complete Symmetric Functions
 - For example, if n = 2, H(t) = (1 - x_1t)^(-1)(1 - x_2t)^(-1) = 1 + (x_1 + x_2)t + (x_1^2 + x_1x_2 + x_2^2)t^2 + ...
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Top images from around the web for Elementary vs Complete Symmetric Functions
Top images from around the web for Elementary vs Complete Symmetric Functions
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Applications and Identities

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  • The can be used to derive various identities and relationships involving elementary and complete symmetric functions
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Top images from around the web for Elementary vs Complete Symmetric Functions
 - For example, the Cauchy identity states that ∏_{i,j} (1 - x_i y_j)^(-1) = Σ_λ s_λ(x)s_λ(y), where s_λ are the Schur functions, which can be expressed in terms of elementary or complete symmetric functions
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Top images from around the web for Elementary vs Complete Symmetric Functions
 - Another example is the [Newton's identities](https://www.fiveableKeyTerm:Newton's_Identities), which relate power sum symmetric functions to elementary symmetric functions: ke_k = Σ_{i=1}^k (-1)^(i-1) p_i e_{k-i}
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Top images from around the web for Elementary vs Complete Symmetric Functions
  • Generating functions provide a powerful tool for studying symmetric functions and their properties
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Top images from around the web for Elementary vs Complete Symmetric Functions
Top images from around the web for Elementary vs Complete Symmetric Functions
Top images from around the web for Elementary vs Complete Symmetric Functions

Relationship Between Symmetric Functions

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Identity Involving Generating Functions

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  • The relationship between elementary and complete symmetric functions is given by the identity: E(t)H(-t) = 1, where E(t) and H(t) are the generating functions for elementary and complete symmetric functions, respectively
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Top images from around the web for Elementary vs Complete Symmetric Functions
 - This identity holds for any number of variables and can be proven using the generating functions and the Cauchy product formula for the product of two power series
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Top images from around the web for Elementary vs Complete Symmetric Functions
 - For example, if n = 2, E(t)H(-t) = (1 + x_1t)(1 + x_2t)(1 + x_1t)^(-1)(1 + x_2t)^(-1) = 1
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Top images from around the web for Elementary vs Complete Symmetric Functions
Top images from around the web for Elementary vs Complete Symmetric Functions
Top images from around the web for Elementary vs Complete Symmetric Functions

Recursive Formula

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  • Expanding the identity E(t)H(-t) = 1 and comparing the coefficients of t^k on both sides leads to the formula: Σ_{i=0}^k (-1)^i e_i h_{k-i} = δ_{k0}, where δ_{k0} is the Kronecker delta (equal to 1 if k = 0 and 0 otherwise)
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Top images from around the web for Elementary vs Complete Symmetric Functions
 - This formula establishes a recursive relationship between elementary and complete symmetric functions
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Top images from around the web for Elementary vs Complete Symmetric Functions
 - For example, if k = 3, the formula gives: e_0h_3 - e_1h_2 + e_2h_1 - e_3h_0 = 0
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Top images from around the web for Elementary vs Complete Symmetric Functions
  • The recursive formula can be used to express elementary symmetric functions in terms of complete symmetric functions and vice versa
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Top images from around the web for Elementary vs Complete Symmetric Functions
 - For example, using the recursive formula, one can show that e_k = Σ_{i=0}^k (-1)^i h_i e_{k-i} and h_k = Σ_{i=0}^k e_i h_{k-i}
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Top images from around the web for Elementary vs Complete Symmetric Functions
 - These expressions allow for the conversion between the two bases of symmetric functions
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Top images from around the web for Elementary vs Complete Symmetric Functions
Top images from around the web for Elementary vs Complete Symmetric Functions
Top images from around the web for Elementary vs Complete Symmetric Functions

Expressing Symmetric Functions

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Top images from around the web for Elementary vs Complete Symmetric Functions
Top images from around the web for Elementary vs Complete Symmetric Functions
Top images from around the web for Elementary vs Complete Symmetric Functions

Fundamental Theorem of Symmetric Functions

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  • The Fundamental Theorem of Symmetric Functions states that every symmetric polynomial can be written as a polynomial in the elementary symmetric functions
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Top images from around the web for Elementary vs Complete Symmetric Functions
 - This theorem guarantees the existence and uniqueness of expressing any symmetric function using the elementary symmetric functions
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Top images from around the web for Elementary vs Complete Symmetric Functions
 - For example, the symmetric function x_1^2 + x_2^2 + x_3^2 can be expressed as e_1^2 - 2e_2, where e_1 and e_2 are the elementary symmetric functions
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Top images from around the web for Elementary vs Complete Symmetric Functions
  • The theorem also holds for complete symmetric functions, meaning that any symmetric function can be expressed as a polynomial in the complete symmetric functions as well
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Top images from around the web for Elementary vs Complete Symmetric Functions
Top images from around the web for Elementary vs Complete Symmetric Functions
Top images from around the web for Elementary vs Complete Symmetric Functions

Process of Expression

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  • Given a symmetric function, one can use algebraic manipulations and the recursive relationship between elementary and complete symmetric functions to express it in terms of either basis
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Top images from around the web for Elementary vs Complete Symmetric Functions
 - The process involves expanding the function and collecting terms with the same degree
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Top images from around the web for Elementary vs Complete Symmetric Functions
 - For example, to express the symmetric function x_1^3 + x_2^3 + x_3^3 using elementary symmetric functions:
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Top images from around the web for Elementary vs Complete Symmetric Functions
      - Expand: x_1^3 + x_2^3 + x_3^3 = (x_1 + x_2 + x_3)^3 - 3(x_1 + x_2 + x_3)(x_1x_2 + x_1x_3 + x_2x_3) + 3x_1x_2x_3
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      - Substitute: (e_1)^3 - 3e_1e_2 + 3e_3
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Top images from around the web for Elementary vs Complete Symmetric Functions
  • The choice of using elementary or complete symmetric functions to express a symmetric function depends on the specific problem and the desired form of the result
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Top images from around the web for Elementary vs Complete Symmetric Functions
 - In some cases, one basis may lead to a simpler or more compact expression than the other
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Top images from around the web for Elementary vs Complete Symmetric Functions
 - For example, the symmetric function x_1^2x_2 + x_1^2x_3 + x_2^2x_1 + x_2^2x_3 + x_3^2x_1 + x_3^2x_2 can be expressed more concisely using complete symmetric functions as h_3 - e_1h_2
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AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.


© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
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