Identity element
In mathematics, the identity element (or neutral element) of a set is a special element of that set. It is special because if it is combined with another element of that set, it does not change the other element.
With addition, the identity element is 0, because adding 0 to some number does not change the number. With multiplication, it is 1.
Further Examples
| set | operation | identity |
|---|---|---|
| real numbers | + (addition) | 0 |
| real numbers | • (multiplication) | 1 |
| real numbers | ab (exponentiation) | 1 (right identity only) |
| m-by-n matrices | + (addition) | zero matrix |
| n-by-n square matrices | • (multiplication) | identity matrix |
| all functions from a set M to itself | ∘ (function composition) | identity map |
| character strings, lists | concatenation | empty string, empty list |
| extended real numbers | minimum/infimum | +∞ |
| extended real numbers | maximum/supremum | -∞ |
| subsets of a set M | ∩ (intersection) | M |
| sets | ∪ (union) | {} (empty set) |
| boolean logic | ∧ (logical and) | ⊤ (truth) |
| boolean logic | ∨ (logical or) | ⊥ (falsity) |
| only two elements {e, f} | * defined by e * e = f * e = e and f * f = e * f = f |
both e and f are left identities, but there is no right or two-sided identity |
