Questions tagged [monoid]
A monoid is an algebraic structure with a single associative binary operation and an identity element.
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Equivalence relation in construction of Grothendieck group
Sorry if this has been asked before but I couldn't find the question I have.
Yesterday I read the wikipedia page for a Grothendieck group. It provided two explicit constructions given a commutative ...
6
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2answers
80 views
A weak cancellation property for monoids
Suppose $M$ is a (commutative) monoid.
Typically the cancellation property is defined as $a + c = b + c \Rightarrow a = b$ for all $a,b,c \in M$.
Recently I was working on a problem where I thought I ...
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1answer
14 views
Converse to a proposition on divisors in commutative monoids
Let $(M,*,1)$ be a commutative monoid. Define the binary relation $R$ on $M$ by $aRb$ iff there exists an $x$ in $M$ such that $a*x=b$. $R$ is the "divides" relation. Since $M$ is a ...
2
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1answer
56 views
Comparability with identity of an ordered semigroup
It is possible to compare any ordered semigroup with $0$:
Comparability with zero of an ordered semigroup
Let's say an ordered semigroup $S$ is comparable with identity if it can be embedded into an ...
4
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1answer
78 views
If the monoid algebra $R[M]$ is finitely generated, then $M$ is a finitely generated monoid.
Consider a commutative, cancellative, torsion-free monoid $M$ and a commutative ring $R.$ If the monoid algebra $R[M]$ is finitely generated as an $R$-algebra, then $M$ is finitely generated.
I am ...
2
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1answer
44 views
Ordered monoid with partial subtraction: Is there a name for this?
I've come across the following structure as being useful to represent various kinds of resources (e.g., fungible things like money, nonfungible things like deeds, etc.), and I was curious if there's a ...
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2answers
40 views
What is the identity in the power set of $\Sigma^*$ as a monoid?
Given an alphabet $\Sigma$, $P (\Sigma^*)$, the power set of $\Sigma^*$, is a monoid, with language concatenation as morphism.
What is the identity: the empty language, or the language consisting of ...
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1answer
17 views
Does string substitution have a definition, similar to the one for string homomorphism in terms of monoid morphism of the free monoid?
string homomorphism is defined in formal language theory as:
A string homomorphism (often referred to simply as a homomorphism in formal language theory) is a string substitution such that each ...
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0answers
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Description of Multicones with Farey intervals
I have a problem in computing and understanding in description of multicones with Farey intervals. Let $M^{*}$ be the monoid On the generators $F_{+}$ , $F_{-}$ operating on words in $A,B$ by the ...
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1answer
52 views
Given any equivalence $\sim$ on $X$ do functions $f:X\to X$ such that $f(u)=f(v)\iff u\sim v$ have a specific name?
Given any equivalence $\sim$ on $X$ do functions $f:X\to X$ such that $f(u)=f(v)\iff u\sim v$ have a specific name?
I know if I replace the image of each element with its pre-image then this is ...
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2answers
192 views
Grothendieck group “commutes” with direct sum
The Grothendieck completion group of a commutative monoid $M$ is the unique (up to isomorphism) pair $\langle \mathcal{G}(M), i_M\rangle$, where $\mathcal{G}(M)$ is an abelian group and $i_M\colon M\...
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0answers
36 views
Comonoids in coslices of a topos
Is it true that there are no nontrivial comonoids (with respect to the cocartesian monoidal structure, of course) in any coslice category of a topos?
Proof that the answer is "Yes" for the case of $...
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0answers
42 views
What is the correct name for a “product function” on a monoid?
Let $W$ be a monoid. A function $f\colon W\rightarrow W$ is a "product function" if $f(w)$ is a product of constants in $W$ and positive integer powers of $w$. It could also be called a "non-...
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0answers
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Is co/complete category of monoid (or commutative monoid)?
Let $Mon$ (resp., $CMon$) be a category of monoids (resp., commutative monoid) whose morphisms are usual monoid homomorphisms (resp., commutative monoid homomorphism). Is this category complete and/or ...
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2answers
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Showing $(M^M, \circ )$ is a monoid.
Show that group $(M^{M},\circ)$ is a monoid with a neutral element $id_{M}$.
$id_{M}$ is defined as "identity mapping" (closest translation that I could have gotten)
$id_{M}: M \rightarrow M, x \...
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1answer
46 views
Monoid - Commutativity
Let $M:=(X, \star)$ be a monoid and let $A\neq \emptyset$ be a set. We define a composition $\hat{\star}$ on $X^A$ by $$\hat{\star}:X^A\times X^A\rightarrow X^A , \ (f,g)\mapsto f\hat{\star} g$$ where ...
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1answer
24 views
Elements and operation of a monoid
Let $(X, \star)$ be a monoid, $e\in X$ the identity element and let $x\star x=e$ for all $x\in X$.
Show the following:
for all $x\in X$ : $x$ is invertible
$\star$ is commutative
$$$$
Could ...
6
votes
2answers
40 views
Reflective subcategories of monoids
An exercise in The Joy of Cats, p. 59, is as follows:
Show that no finite monoid, considered as a category, has a proper reflective subcategory.
The obvious idea is to let $r : \cdot \to \cdot$ be ...
5
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3answers
103 views
Coproduct of free monoids.
Consider the following diagram, where $A,B$ are sets, $A+B$ is their disjoint union, and $M(X)$ is the free monoid on $X$ for $X=A, B, A+B$. I want to prove that $M(A+B)$ is the coproduct of $M(A)$ ...
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Reduced decompositions of elements of an amalgamated sum of monoids
Let $(M_i)_{i\in I}$ be a family of monoids, $A$ a monoid and $(h_i:A\rightarrow M_i)_{i\in I}$ a family of homomorphisms.
Let $M$ be the sum of the family $(M_i)_{i\in I}$ amalgamated by $A$. Let $\...
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0answers
25 views
Factorization of the identity element of the free monoid
Let $X$ be a set and let $\text{Mo}(X)=\bigcup_{n\in\mathbb{N}} X^{[1,n]}$. Then $\text{Mo}(X)$ together with the law $(w,w')\mapsto ww'$, where $ww'$ denotes the juxtaposition of the sequences $w$ ...
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1answer
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The empty sequence is the identity element of the free monoid constructed on a set X
Let $X$ be a set. Let $w=(x_i)_{1\leq i\leq m}$ and $w'=(x'_j)_{1\leq j\leq n}$ for some $m,n\in\mathbb{N}$. The composition of $w$ and $w'$, denoted by $ww'$, is the family $(y_k)_{1\leq k\leq m+n}$ ...
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1answer
25 views
Quotients of semilattices (?)
I want to build some structure and I think a semilattice is the best way to do so, but don't know for sure.
I have some (non-empty) set $X$ with some distinguished elements $a,b,c,d$ and $e$. I know ...
3
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0answers
34 views
Each magma $M$ is associated with monoids $\mathcal{L}(M)$ and $\mathcal{R}(M)$. What are these called, and have they been studied?
Let $X$ denote a magma. Then $\mathrm{List}(X)$ is a monoid equipped with both a left and a right action on $X$, where the actions are defined in the obvious way. To illustrate these actions, suppose ...
5
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4answers
59 views
Group congruences: If the operation is preserved, do we get $a\sim b$ $\Rightarrow$ $a^{-1}\sim b^{-1}$?
Let $(G,*)$ be a group. Let $\sim$ be an equivalence relation such that
$$(\forall a,a',b,b'\in G)a\sim a', b\sim b' \Rightarrow a*b\sim a'*b'. \tag{*}$$
I.e., the equivalence relation $\sim$ respects ...
0
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1answer
24 views
Commutative and additive monoids [closed]
I am unfamiliar with higher levels of topology and number theory but find myself working on a project in which I need an understanding of some of those topics. Is anyone able to provide information on ...
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1answer
53 views
Does $\gcd(I)=1$ imply the monoid generated by $I$ is $\mathbb{N}$ minus finitely many numbers?
This is true if $I=\{a_1,\dots,a_n\}$ is a finite set of positive integers. Namely, if $\gcd(a_1,\dots,a_n)=1$, then for all sufficiently large $N$ there is a non-negative integer solution $(k_1,\dots,...
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2answers
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Should I say Abelian monoïd or commutative monoïd
I usually say, "Abelian group" rather than "commutative group", not sure if that's because I studied in the United states during the 1980s. But it seems people in Europe say, "commutative monoïd". ...
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2answers
54 views
example of monoids
An element $x$ of a semigroup $S$ is called regular provided that there exists $y\in S$ such that $xyx=x$. $S$ is called regular if all its elements are regular. Let $S$ be a monoid with identity ...
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0answers
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Difference of congruence relations concerning monoid homomorphisms
So there is the congruence relation defined for an "amalgamated sum" of monoid morphisms that is generated by $((a,b),(a',b'))\in (N_1\oplus N_2)\times (N_1\oplus N_2)$ such that there is an element $...
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0answers
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refering something as “non-linear” when there is no underlying linear structure
Can I talk about a non-linear shape functional. I understand a shape functional $J$ as some mapping that takes a shape and returns a real (or complex) value.
I would like to talk about a non-linear ...
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2answers
40 views
generators and monoid homomorphisms
Is it true that any homomorphism $f: M \to N$ between two monoids $M$ and $N$ maps generators of $M$ to generators of $N$? I am having trouble proving it to myself.
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1answer
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Monoid morphisms
For one of my computer science classes, I am asked to give examples of monoid morphisms for the following morphisms.
However, I really don't know how to approach it since we haven't worked with ...
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1answer
26 views
Monoid that is idempotent induces partial ordering
Given a commutative monoid $(M,0,\oplus)$.
Then we can define an ordering on $M$ by
$a\geq b :\Leftrightarrow \exists c: a=b\oplus c$.
The relation is then transitive and reflexive. The claim is now ...
2
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1answer
91 views
Exponents in $\mathbf{Sets}^{G^{{\rm op}}}$ for an arbitrary group $G$.
This is Exercise I.5(b) of Mac Lane and Moerdijk's, "Sheaves in Geometry and Logic [. . .]." According to Approach0, it is new to MSE.
The Details:
From p. 17 ibid. . . .
Definition 1: ...
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1answer
49 views
proving result from monoid theory
An element $x$ of a monoid $M$ is called an atom if $x \neq 1$ and $x=ab$, for some $a,b \in M$, implies that $x=a$ or $x=b$. Furthermore, $M$ is said to be atomic if every element $a \in M$ can be ...
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0answers
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The category of monoid objects
Let $C$ be a cartesian category.
We can form the category $\text{Mon}(C)$ of monoid objects in $C$. If $C$ is $\text{Set}$, this category is the category of monoids. If $C$ is $\text{Mon}$ (the ...
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3answers
181 views
How does Universal Mapping Property encode “no-junk” and “no-noise” in free monoid?
I am going trough the "Category Theory" book by Steve Awodey.
In the "1.7 Free categories" chapter the author introduces the following algebraic definition of free monoid:
A ...
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1answer
44 views
Construction of enveloping group of a monoid
Let $M$ be a monoid and let $G$ be the group which it generates. $G$ can be described as the group obtained from $M$ by adjoining formal inverses. Despite this simple description, I am trying to ...
3
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1answer
141 views
Group freely generated by monoid
There are several ways to define the group freely generated by a monoid, all of which (necessarily) produce isomorphic groups. One way starts with a presentation of the monoid, and simply reinterprets ...
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1answer
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Proving Transitivity for the equivalence relation: $\langle a,b \rangle \sim \langle c,d \rangle \iff a+_{\mathbb{N}}d=b+_{\mathbb{N}}c$
For any $a,b,c,d \in \mathbb N$, I am trying to demonstrate that $\langle a,b \rangle \sim \langle c,d \rangle \iff a+_{\mathbb{N}}d=b+_{\mathbb{N}}c$.
I am trying to do this without invoking the ...
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0answers
72 views
left and right Ore
A monoid $M$ satisfies the left Ore condition if for all $a,b \in M$ there exist $c,d \in M$ such that $ca=db$. Suppose, in addition, $ac=bc$ implies that there exists an element $d \in M$ such that $...
2
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1answer
34 views
Commutative monoids $(S,+,0)$ where $\forall x,y \in S$ $x+ y = x$ implies $y = 0$?
Is there a definition/name for such monoids?
Has any theory been developed for such monoids?
Are there any any references/links delving into this with perhaps additional axioms?
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1answer
30 views
Shortest path length in graph generated by free monoidal multiplication
Let $X^*$ be the free monoid on a set $X$. Now define the graph with vertices $X^*$, and the edges $E \subset (X^*)^2$ recursively as follows:
$(1, 1) \in E$.
If $(w, w') \in E$, then $(w', w' \cdot ...
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1answer
38 views
Unique function whose product is a unique monoid element?
Let $P$ be the free monoid on a set $X$. Given $p \in P$, this induces a unique function $f : [n] \to X$ with $\prod_{i\in[n]} f(i) = p$. Does this function have a name?
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1answer
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Formula for $M_{p^k} = \{ x \in \Bbb{Z} : x^2 = 1 \pmod {p^k}\}$?
Let $M_n = \{ x \in \Bbb{Z}: x^2 = 1 \pmod n\}$. It is a multiplicative submonoid of $\Bbb{Z}$.
When $\gcd(a,b) = 1$ then we have:
$$
x^2 = 1\pmod {a,b} \iff \\
x^2 = 1\pmod {ab}
$$
by the Chinese ...
1
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1answer
92 views
Quotienting by kernel of a monoid homomorphism in the integers.
If $f : \Bbb{Z}^{\times} \to (\Bbb{Z}/n)^{\times}$ is a monoid hom, then is $\Bbb{Z}^{\times} / \ker f \approx \operatorname{im} f$? For example $f(x) = x^2$.
That is can we quotient a monoid by the ...
2
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1answer
43 views
Group of mappings
Is there a group $G$ of mappings $X \to X$ that has a non-bijective map in it? I mean, for each element of G, it must has its inverses at right and left, and those must be the same, so the element is ...
0
votes
1answer
26 views
Union of submonoids of a monoid
Let $E$ be a monoid. I know that the intersection of any
family of submonoids of $E$ is again a submonoid of $E$. Under what conditions is the union of an arbitrary family of submonoids of $E$ a ...
0
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2answers
70 views
There is no natural polynomial map (other than $1$) that can eventually leave the $k$-semiprimes behind.
Let $F(S)$ be the free commutative monoid on countably many symbols $S$. Then it's obvious that $F(S) = \{1\} \uplus S \uplus S^2 \uplus \dots$
One can take $S = $ the prime numbers in $\Bbb{N}$ in ...
