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In abstract algebra, the free monoid on a set is the monoid whose elements are all the finite sequences (or strings) of zero or more elements from that set, with string concatenation as the monoid operation and with the unique sequence of zero elements, often called the empty string and denoted by ε or λ, as the identity element.
This monoid is denoted Σ ∗ and is called the free monoid over Σ. It is not commutative if Σ has at least two elements. Given any monoid M, the opposite monoid M op has the same carrier set and identity element as M, and its operation is defined by x • op y = y • x. Any commutative monoid is the opposite monoid of itself.
The Kleene star is defined for any monoid, not just strings. More precisely, let ( M , ⋅) be a monoid, and S ⊆ M . Then S * is the smallest submonoid of M containing S ; that is, S * contains the neutral element of M , the set S , and is such that if x , y ∈ S * , then x ⋅ y ∈ S * .
Conversely, the set of isomorphism classes (if such a thing makes sense) of a monoidal category is a monoid w.r.t. the tensor product. Any commutative monoid (,,) can be realized as a monoidal category with a single object. Recall that a category with a single object is the same thing as an ordinary monoid.
A simpler example are the free monoids. The free monoid on a set X, is the monoid of all finite strings using X as alphabet, with operation concatenation of strings. The identity is the empty string. In essence, the free monoid is simply the set of all words, with no equivalence relations imposed.
The monoid is then presented as the quotient of the free monoid (or the free semigroup) by these relations. This is an analogue of a group presentation in group theory. As a mathematical structure, a monoid presentation is identical to a string rewriting system (also known as a semi-Thue system). Every monoid may be presented by a semi-Thue ...
The bicyclic semigroup is the quotient of the free monoid on two generators p and q by the congruence generated by the relation p q = 1. Thus, each semigroup element is a string of those two letters, with the proviso that the subsequence "p q" does not appear. The semigroup operation is concatenation of strings, which is clearly associative.
The non-negative integers form a cancellative monoid under addition. Each of these is an example of a cancellative magma that is not a quasigroup. Any free semigroup or monoid obeys the cancellative law, and in general, any semigroup or monoid that embeds into a group (as the above examples clearly do) will obey the cancellative law.