# Symbols:Greek/Sigma

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## Sigma

The $18$th letter of the Greek alphabet.

Minuscules: $\sigma$ and $\varsigma$
Majuscule: $\Sigma$

The $\LaTeX$ code for $\sigma$ is \sigma .
The $\LaTeX$ code for $\varsigma$ is \varsigma .

The $\LaTeX$ code for $\Sigma$ is \Sigma .

## Event Space

$\Sigma$

Let $\mathcal E$ be an experiment.

The event space of $\mathcal E$ is usually denoted $\Sigma$ (Greek capital sigma), and is the set of all outcomes of $\mathcal E$ which are interesting.

## Summation

Let $\struct {S, +}$ be an algebraic structure where the operation $+$ is an operation derived from, or arising from, the addition operation on the natural numbers.

Let $\tuple {a_1, a_2, \ldots, a_n} \in S^n$ be an ordered $n$-tuple in $S$.

The composite is called the summation of $\tuple {a_1, a_2, \ldots, a_n}$, and is written:

$\displaystyle \sum_{j \mathop = 1}^n a_j = \tuple {a_1 + a_2 + \cdots + a_n}$

The $\LaTeX$ code for $\displaystyle \sum_{j \mathop = 1}^n a_j$ is \displaystyle \sum_{j \mathop = 1}^n a_j .

The $\LaTeX$ code for $\displaystyle \sum_{1 \mathop \le j \mathop \le n} a_j$ is \displaystyle \sum_{1 \mathop \le j \mathop \le n} a_j .

The $\LaTeX$ code for $\displaystyle \sum_{\map \Phi j} a_j$ is \displaystyle \sum_{\map \Phi j} a_j .

## Sigma Function

$\map \sigma n$

Let $n$ be an integer such that $n \ge 1$.

The sigma function $\map \sigma n$ is defined on $n$ as being the sum of all the positive integer divisors of $n$.

That is:

$\displaystyle \map \sigma n = \sum_{d \mathop \divides n} d$

where $\displaystyle \sum_{d \mathop \divides n}$ is the sum over all divisors of $n$.

The $\LaTeX$ code for $\map \sigma n$ is \map \sigma n .

## Surface Charge Density

$\sigma$

Denotes the surface charge density of a given body:

$\sigma = \dfrac q A$

where:

$q$ is the body's electric charge;
$A$ is the body's area.

## Area Density

$\sigma$

Sometimes used, although $\rho_A$ (Greek letter rho) is more common, to denote the area density of a given two-dimensional body:

$\sigma = \dfrac m A$

where:

$m$ is the body's mass
$A$ is the body's area.

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The $\LaTeX$ code for $\sigma$ is \sigma .