Symmetric function

Short description: Function that is invariant under all permutations of its variables

In mathematics, a function of [math]\displaystyle{ n }[/math] variables is symmetric if its value is the same no matter the order of its arguments. For example, a function [math]\displaystyle{ f\left(x_1,x_2\right) }[/math] of two arguments is a symmetric function if and only if [math]\displaystyle{ f\left(x_1,x_2\right) = f\left(x_2,x_1\right) }[/math] for all [math]\displaystyle{ x_1 }[/math] and [math]\displaystyle{ x_2 }[/math] such that [math]\displaystyle{ \left(x_1,x_2\right) }[/math] and [math]\displaystyle{ \left(x_2,x_1\right) }[/math] are in the domain of [math]\displaystyle{ f. }[/math] The most commonly encountered symmetric functions are polynomial functions, which are given by the symmetric polynomials.

A related notion is alternating polynomials, which change sign under an interchange of variables. Aside from polynomial functions, tensors that act as functions of several vectors can be symmetric, and in fact the space of symmetric [math]\displaystyle{ k }[/math]-tensors on a vector space [math]\displaystyle{ V }[/math] is isomorphic to the space of homogeneous polynomials of degree [math]\displaystyle{ k }[/math] on [math]\displaystyle{ V. }[/math] Symmetric functions should not be confused with even and odd functions, which have a different sort of symmetry.

## Contents

* 1 Symmetrization
* 2 Examples
* 3 Applications
* 3.1 U-statistics
* 4 See also
* 5 References

## Symmetrization

Main page: Symmetrization

Given any function [math]\displaystyle{ f }[/math] in [math]\displaystyle{ n }[/math] variables with values in an abelian group, a symmetric function can be constructed by summing values of [math]\displaystyle{ f }[/math] over all permutations of the arguments. Similarly, an anti-symmetric function can be constructed by summing over even permutations and subtracting the sum over odd permutations. These operations are of course not invertible, and could well result in a function that is identically zero for nontrivial functions [math]\displaystyle{ f. }[/math] The only general case where [math]\displaystyle{ f }[/math] can be recovered if both its symmetrization and antisymmetrization are known is when [math]\displaystyle{ n = 2 }[/math] and the abelian group admits a division by 2 (inverse of doubling); then [math]\displaystyle{ f }[/math] is equal to half the sum of its symmetrization and its antisymmetrization.

## Examples

* Consider the real function [math]\displaystyle{ f(x_1,x_2,x_3) = (x-x_1)(x-x_2)(x-x_3). }[/math] By definition, a symmetric function with [math]\displaystyle{ n }[/math] variables has the property that [math]\displaystyle{ f(x_1,x_2,\ldots,x_n) = f(x_2,x_1,\ldots,x_n) = f(x_3,x_1,\ldots,x_n,x_{n-1}), \quad \text{ etc.} }[/math] In general, the function remains the same for every permutation of its variables. This means that, in this case, [math]\displaystyle{ (x-x_1)(x-x_2)(x-x_3) = (x-x_2)(x-x_1)(x-x_3) = (x-x_3)(x-x_1)(x-x_2) }[/math] and so on, for all permutations of [math]\displaystyle{ x_1, x_2, x_3. }[/math]
* Consider the function [math]\displaystyle{ f(x,y) = x^2 + y^2 - r^2. }[/math] If [math]\displaystyle{ x }[/math] and [math]\displaystyle{ y }[/math] are interchanged the function becomes [math]\displaystyle{ f(y,x) = y^2 + x^2 - r^2, }[/math] which yields exactly the same results as the original [math]\displaystyle{ f(x, y). }[/math]
* Consider now the function [math]\displaystyle{ f(x,y) = ax^2+by^2-r^2. }[/math] If [math]\displaystyle{ x }[/math] and [math]\displaystyle{ y }[/math] are interchanged, the function becomes [math]\displaystyle{ f(y,x) = ay^2 + bx^2 - r^2. }[/math] This function is not the same as the original if [math]\displaystyle{ a \neq b, }[/math] which makes it non-symmetric.

## Applications

### U-statistics

Main page: U-statistic

In statistics, an [math]\displaystyle{ n }[/math]-sample statistic (a function in [math]\displaystyle{ n }[/math] variables) that is obtained by bootstrapping symmetrization of a [math]\displaystyle{ k }[/math]-sample statistic, yielding a symmetric function in [math]\displaystyle{ n }[/math] variables, is called a U-statistic. Examples include the sample mean and sample variance.

## See also

* Alternating polynomial
* Elementary symmetric polynomial
* Even and odd functions - Functions such that f(–x) equals f(x) or f(–x)
* Exchangeable random variables - Concept in statistics
* Quasisymmetric function
* Ring of symmetric functions
* Symmetrization
* Vandermonde polynomial

## References

* F. N. David, M. G. Kendall & D. E. Barton (1966) Symmetric Function and Allied Tables, Cambridge University Press .
* Joseph P. S. Kung, Gian-Carlo Rota, & Catherine H. Yan (2009)  The Rota Way, §5.1 Symmetric functions, pp 222–5, Cambridge University Press, ISBN 978-0-521-73794-4.

* v
* t
* e

Tensors

Glossary of tensor theory

Scope|

| Mathematics|

* coordinate system
* multilinear algebra
* Euclidean geometry
* tensor algebra
* dyadic algebra
* differential geometry
* exterior calculus
* tensor calculus

|

* Physics
* Engineering

* continuum mechanics
* electromagnetism
* transport phenomena
* general relativity
* computer vision

Notation|

* index notation
* multi-index notation
* Einstein notation
* Ricci calculus
* Penrose graphical notation
* Voigt notation
* abstract index notation
* tetrad (index notation)
* Van der Waerden notation

Tensor
definitions|

* tensor (intrinsic definition)
* tensor field
* tensor density
* tensors in curvilinear coordinates
* mixed tensor
* antisymmetric tensor
* symmetric tensor
* tensor operator
* tensor bundle
* two-point tensor

Operations|

* tensor product
* exterior product
* tensor contraction
* transpose (2nd-order tensors)
* raising and lowering indices
* Hodge star operator
* covariant derivative
* exterior derivative
* exterior covariant derivative
* Lie derivative

Related
abstractions|

* dimension
* basis
* vector, vector space
* multivector
* covariance and contravariance of vectors
* linear transformation
* matrix
* spinor
* Cartan formalism
* differential form
* exterior form
* connection form
* geodesic
* manifold
* fiber bundle
* Levi-Civita connection
* affine connection

Notable tensors|

| Mathematics|

* Kronecker delta
* Levi-Civita symbol
* metric tensor
* nonmetricity tensor
* Christoffel symbols
* Ricci curvature
* Riemann curvature tensor
* Weyl tensor
* torsion tensor

|

Physics|

* moment of inertia
* angular momentum tensor
* spin tensor
* Cauchy stress tensor
* stress–energy tensor
* EM tensor
* gluon field strength tensor
* Einstein tensor
* metric tensor (GR)

Mathematicians|

* Leonhard Euler
* Carl Friedrich Gauss
* Augustin-Louis Cauchy
* Hermann Grassmann
* Gregorio Ricci-Curbastro
* Tullio Levi-Civita
* Jan Arnoldus Schouten
* Bernhard Riemann
* Elwin Bruno Christoffel
* Woldemar Voigt
* Élie Cartan
* Hermann Weyl
* Albert Einstein

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