Sigma (capital σ, lowercase σ, Chinese transliteration: Sigma) is the eighteenth Greek letter. In Greek, if the last letter of a word is lowercase sigma, what should this letter be written as? This letter is also called final sigma(Unicode: U+03C2). In modern Greek numerals, it stands for 6.

σ is used for:

● Mathematical summation symbol (also called sum number)

● Sima in Locke X, the old enemy of X and zero.

Lowercase σ is used for:

Sigma bond, a chemical bond formed by the "head-to-head" of atomic orbits.

● Statistical standard deviation

The symbol "σ" was first used by Euler (1707- 1783) in 1755. This symbol comes from the Greek prefix (addition), and "σ" is the capital of σ.

For example: σ an = a 1+a2+...+ An

∑ is a simple symbol of summation of series, followed by k 2 is a general term formula, the following k= 1 is the number of terms at the beginning of the initial term, and the above n is the number of terms at the end.

n

∑k^2= 1^2+2^2+……+n^2( 1)

k= 1

n

∑(2k+ 1)= 3+5+……+(2n+ 1)(2)

k= 1

Then (1)+(2)= 1

n

∑(k+ 1)^2=2^2+3^2+……+(n+ 1)^2

k= 1

The extension of the famous binomial theorem can be expressed as

n

∑C(n,k)a^(n-k)b^k.

k=0

This shows the possibility of application, and its application is quite flexible.