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Let Set denote the category of sets.

Let $T:$ Set $\to$ Set be the functor that sends a set $X$ to the set of finite words on $X$.

That is, $TX = \{[x_m,..,x_1] : m = 0,1,2,3..., x_i \in X\}$

$T$ can be considered a monad on Set with multiplication given by concatenation, and the unit by $x \to [x]$.

I'm trying to show explicitly that the category of $T$ algebras on Set is equivalent to the category of monoids (unital).

To do this we can define a functor $F:$ Mon $\to$ Alg$_T$(Set) by $F(m) = (m,a)$ where $a: Tm \to m$ is s.t $a([f_n,..,f_1]) = f_n...f_1$.

I have shown that this functor is fully faithful.

However I'm not sure I understand if it is essentially surjective.

Letting $(X,a_X) \in$ Alg$_T$(Set). I thought of defining a monoid $m$ as $TX$, where the action is concatenation, and the unit is simply the empty word.

Doing this yields a natural map in Alg$_T$(Set) from $TX$ to $X$, namely $a_X$. Clearly this map is onto because $a_X([x]) = x$ by definition of $(X, a_X)$ being an algebra.

But is this map injective?

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  • $\begingroup$ Why would you want this map to be injective ? $\endgroup$ Dec 26 '18 at 14:58
  • $\begingroup$ @Max because I need to show that $F$ is essentially surjective, so I need that $(X,a_X)$ is in the essential range. $\endgroup$
    – Mariah
    Dec 26 '18 at 15:09
  • $\begingroup$ Oh right I didn't understand what you were trying to do. But it isn't injective. You want to see that $X$ is the underlying set of a monoid. To define the multiplication of $x,y$, put $x\times y := a_X([x,y])$ $\endgroup$ Dec 26 '18 at 15:13
  • $\begingroup$ @Max ok, but then what do we consider the identity element? It should be a word in $X$ by your statement, but this doesn't make sense. This structure you suggested allows for writing $a_X([x]) = x$ indeed, but don't we need the identity to be in $X$? $\endgroup$
    – Mariah
    Dec 26 '18 at 16:25
  • $\begingroup$ $a_X([])$ will be the identity. Let me write an answer it will be clearer $\endgroup$ Dec 26 '18 at 16:46
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You're starting from a $T$-algebra $(X,a_X)$, and you wish to show that it is isomorphic to $F(M)$ for some monoid $M$.

In particular, since $F(M)$ is of the form $(M,h)$ for $h:TM\to M, [m_1,...,m_n]\mapsto m_1...m_n$, then if there is such an isomorphism $f:X\to M$ and if $x,y\in X$, then $h\circ T(f)([x,y]) = h([f(x),f(y)])=f(x)f(y)$ and $h\circ T(f)([x,y] ) = f\circ a_X([x,y])$.

Considering that $f$ is a bijection and thus identifying $M$ and $X$ with it, we get a monoid structure on $X$ such that $a_X([x,y]) = xy$.

Thus now if we want to find $M$, we know what we should do : define multiplication on $X$ by the formula $xy := a_X([x,y])$; and check that this makes $X$ into a monoid such that $F(X) \simeq (X,a_X)$ (actually it will be an equality !)

To prove that $X$ is indeed a monoid with this operation, you will need to use the different properties of the monad and axioms of $T$-algebra.

Associativity will follow from the associativity axiom of $T$-algebras that says that this should commute :

$\require{AMScd} \begin{CD} TTX @>{\mu_X}>> TX\\ @V{T(a_X)}VV @VV{a_X}V\\ TX @>>{a_X}> X \end{CD}$

and the unit will be $a_X([ ])$ and the fact that this is a unit will follow from the associativity axiom for $T$-algebras, together with the unit axiom of $T$-algebras that says that this should commute :

$\require{AMScd} \begin{CD} X @>{\eta_X}>> TX\\ @V{id_X}VV @VV{a_X}V\\ X @>>{id_X}> X \end{CD}$

You then need to prove that $a_X([x_1,...,x_n]) = x_1...x_n$ but this will follow from associativity again. I'll let you check the details.

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  • $\begingroup$ To show that F is fully faithful do you not need to verify injectivity/surjectivity on the Hom sets as well? If I understand your answer correctly, you have only shown that F is surjective when viewed as a map of objects... $\endgroup$
    – gen
    Dec 31 '18 at 16:26
  • $\begingroup$ @gen : The OP states "I have shown that this functor is fully faithful" $\endgroup$ Dec 31 '18 at 16:35
  • $\begingroup$ Oh I see, thanks for clarifying! $\endgroup$
    – gen
    Dec 31 '18 at 16:36

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