Commutative or abelian monoid . In Algebra it is said of the algebraic structure formed by the pair <G, *> , such that it is a monoid and * is commutative .
Let be a set G and the binary operation * defined as * (x, y) = z or better x * y = z and each of the following axioms is satisfied :
- Closing: . * is closed.
- Associativity: For all x , y , z of G , (x * y) * z = x * (y * z) .
- Existence of neutral: There is one and only one element e of G such that for all x of G it is true that x * e = e * x = x . and it is called I neutral for * in G .
- Commutativity: For all x and y of G , it is true that x * y = y * x .
It is said that G with the operation * is commutative or Abelian monoid .
In other words:
- <G, *>is monoide commutative if and only if <G, *> is monoid and * is commutative for all elements of G .
- The integersand the sum make up an abelian monoid since the sum is closed and associative, 0 is neutral.
- Natural numbersand product are also a commutative monoid where 1 is the neutral of multiplication .
- The naturals and the sum do not form a commutative monoid because although the sum is symmetric, the axiom of the existence of the neutral that invalidates the structure as a monoid is violated.
- The stringand the concatenation form a commutative monoid having the empty string by neutral.
- Let any algebraic group be <M, *>, it is also a monoid.