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A semigroup is a set $S$ together with a binary operation that is associative. Examples of semigroups are the set of finite strings over a fixed alphabet (under concatenation) and the positive integers (under addition, maximum, or minimum). A monoid is a semigroup with a neutral element. Of course, any group is also a monoid/semigroup.

7 votes
1 answer
441 views

Which monoids have a faithful irreducible representation?

Let $*$ be a binary operation on a set $M$, with an identity element $e\in M$. A monoid representation of $(M,*,e)$ is a map $\delta:M\to (S\to S)$ for some set $S$, such that $\delta(e)=\mathrm{id}_S …
9 votes
Accepted

Is the equational theory of groups axiomatized by the associative law?

Yes. It suffices to show that any free semigroup embeds in a group. For this I refer you to MO question 3235: Let $F$ be a free semigroup (say, $2$-generated) which is embedded in a group $G$, an …
Bjørn Kjos-Hanssen's user avatar
8 votes
Accepted

Define Turing machine with algebraic concepts/structures

Yes, there is now Pavlovic's characterization of Turing computability in terms of the monoidal computer, based on monoidal categories. http://arxiv.org/abs/1208.5205
Bjørn Kjos-Hanssen's user avatar
7 votes
Accepted

Über theorem on unavoidable patterns?

According to the 2013 paper "Computing the Partial Word Avoidability Indices of Ternary Patterns" by Blanchet-Sadri, Lohr, and Scott, The problem of deciding whether a given pattern is avoidable h …
Bjørn Kjos-Hanssen's user avatar