Monoid . In Algebra it is said of the algebraic structure formed by the pair <G, *> , such that G is a non-empty set and * is a binary operation ; then it is true that * is closed and associative and that the neutral element exists in G for * .

In the case that <G, *> is monoid and the operation is commutative, it is said to be a commutative or abelian monoid .


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 :

  1. Closing: . * is closed.
  2. Associativity: For all x , y , z of G , (x * y) * z = x * (y * z) .
  3. 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 .

It is said that G with the operation * is a monoid .


  • The integersand the sum make up a monoid since the sum is closed and associative, 0 is neutral.
  • The natural numbersand the product are also a monoid where 1 is the neutral of the multiplication .
  • The stringand the concatenation form a monoid having the empty string by neutral.
  • Let any algebraic group be <M, *>, it is also a monoid.


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