Symmetrization

In mathematics, symmetrization is a process that converts any function in n variables to a symmetric function in n variables. Similarly, anti-symmetrization converts any function in n variables into an antisymmetric function.

Two variables

Let $S$ be a set and $A$ an abelian group. A map $\alpha \colon S\times S\to A$ $\alpha$ is called symmetric if $\alpha (s,t)=\alpha (t,s)$ for all $s,t\in S$ .

The symmetrization of a map $\alpha \colon S\times S\to A$ is the map $(x,y)\mapsto \alpha (x,y)+\alpha (y,x)$ .

Similarly, the anti-symmetrization or skew-symmetrization of a map $\alpha \colon S\times S\to A$ is the map $(x,y)\mapsto \alpha (x,y)-\alpha (y,x)$ .

The sum of the symmetrization and the anti-symmetrization of a map α is 2α. Thus, away from 2, meaning if 2 is invertible, such as for the real numbers, one can divide by 2 and express every function as a sum of a symmetric function and an anti-symmetric function.

The symmetrization of a symmetric map is its double, while the symmetrization of an alternating map is zero; similarly, the anti-symmetrization of a symmetric map is zero, while the anti-symmetrization of an anti-symmetric map is its double.

Bilinear forms

The symmetrization and anti-symmetrization of a bilinear map are bilinear; thus away from 2, every bilinear form is a sum of a symmetric form and a skew-symmetric form, therefore there is no difference between a symmetric form and a quadratic form.

At 2, not every form can be decomposed into a symmetric form and a skew-symmetric form. For instance, over the integers, the associated symmetric form (over the rationals) may take half-integer values, while over $\mathbf {Z} /2\mathbf {Z} ,$ a function is skew-symmetric if and only if it is symmetric (as 1 = −1).

This leads to the notion of ε-quadratic forms and ε-symmetric forms.

Representation theory

In terms of representation theory:

exchanging variables gives a representation of the symmetric group on the space of functions in two variables,
the symmetric and anti-symmetric functions are the subrepresentations corresponding to the trivial representation and the sign representation, and
symmetrization and anti-symmetrization map a function into these subrepresentations – if one divides by 2, these yield projection maps.

As the symmetric group of order two equals the cyclic group of order two ( $\mathrm {S} _{2}=\mathrm {C} _{2}$ ), this corresponds to the discrete Fourier transform of order two.

n variables

More generally, given a function in n variables, one can symmetrize by taking the sum over all $n!$ permutations of the variables,[1] or anti-symmetrize by taking the sum over all $n!/2$ even permutations and subtracting the sum over all $n!/2$ odd permutations (except that when n ≤ 1, the only permutation is even).

Here symmetrizing a symmetric function multiplies by $n!$ – thus if $n!$ is invertible, such as when working over a field of characteristic $0$ or $p>n$ , then these yield projections when divided by $n!$ .

In terms of representation theory, these only yield the subrepresentations corresponding to the trivial and sign representation, but for $n>2$ there are others – see representation theory of the symmetric group and symmetric polynomials.

Bootstrapping

Given a function in k variables, one can obtain a symmetric function in n variables by taking the sum over k-element subsets of the variables. In statistics, this is referred to as bootstrapping, and the associated statistics are called U-statistics.

Notes

Hazewinkel (1990), p. 344

References

Hazewinkel, Michiel (1990). Encyclopaedia of mathematics: an updated and annotated translation of the Soviet "Mathematical encyclopaedia". Encyclopaedia of Mathematics. 6. Springer. ISBN 978-1-55608-005-0.

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[1] Hazewinkel (1990), p. 344