An e such that for all x∈S, e*x=x*e=x

(chk) ∀a, ∃e such that a*e = e*a = a를 만족하는. 이 때 e가 항등원.

덧셈,addition의 항등원 = 0 (∵ x+0=0+x=x)
곱셈,multiplication의 항등원 = 1
...
각각
맞는지?

행렬곱셈,matrix_multiplication 의 항등원?은 - chk
항등행렬,identity_matrix = 단위행렬,unit_matrix I or E

left identity
left identity element
https://proofwiki.org/wiki/Definition:Left_Identity
right identity
right identity element
https://proofwiki.org/wiki/Definition:Right_Identity
two-sided identity
two-sided identity element
https://proofwiki.org/wiki/Definition:Identity_Element
https://planetmath.org/leftidentityandrightidentity
... left identity right identity

ADDIDENTITYHERE

비교: 역원,inverse_element

AKA 줄여서 identity (저기엔 항등식,identity의 뜻도 있음)
AKA neutral element

mklink
항등함수,identity_function

Twins:
수학백과: 항등원
https://planetmath.org/identityelement - syn. neutral element
https://en.citizendium.org/wiki/Identity_element
https://encyclopediaofmath.org/wiki/Identity_element - neutral element, unit element, zero element
Identity_element
https://ncatlab.org/nlab/show/identity element - "sometimes also called neutral element"
https://groupprops.subwiki.org/wiki/Identity_element

https://everything2.com/title/identity element
identity_element

// forked from 항등식,identity... 항등- (identity-) 관련으로 분류? see 항등식.

Up: 대수구조,algebraic_structure 군론,group_theory? 항등성,identity 원소,element