# Elementary symmetric sum

An elementary symmetric sum is a type of summation.

## Definition

The $k$-th elmentary symmetric sum of a set of $n$ numbers is the sum of all products of $k$ of those numbers ( $1 \leq k \leq n$). For example, if $n = 4$, and our set of numbers is $\{a, b, c, d\}$, then:

1st Symmetric Sum = $e_1 = a+b+c+d$

2nd Symmetric Sum = $e_2 = ab+ac+ad+bc+bd+cd$

3rd Symmetric Sum = $e_3 = abc+abd+acd+bcd$

4th Symmetric Sum = $e_4 = abcd$

## Notation

The first elmentary symmetric sum of $f(x)$ is often written $\sum_{sym}f(x)$. The $n$th can be written $\sum_{sym}^{n}f(x)$

## Uses

Any symmetric sum can be written as a polynomial of the elmentary symmetric sum functions. For example, $x^3 + y^3 + z^3 = (x+y+z)(x^2 + y^2 + z^2 - xy - yz - xz) + 3xyz = e_1^3 - 3e_1e_2 + 3e_3$. This is often used to solve systems of equations involving power sums, combined with Vieta's.

Elmentary symmetric sums show up in Vieta's formulas. In a monic polynomial, the coefficient of the $x^1$ term is $e_1$, and the coefficient of the $x^k$ term is $e_k$.