diff --git a/content/separated-nominal-automata.tex b/content/separated-nominal-automata.tex index 9864c70..7cb396d 100644 --- a/content/separated-nominal-automata.tex +++ b/content/separated-nominal-automata.tex @@ -159,11 +159,23 @@ Moreover, recall that every $\sb$-set is also a $\perm$-set; the associated noti In particular, this means that the forgetful functor restricts to $U \colon \sbnom \to \permnom$. \startlemma[reference=lem:GM-support] -(Theorem 4.8 from \citet[Gabbay07]) +(Theorem 4.8 from \citenp[Gabbay07]) Let $X$ be a nominal $\sb$-set, $x \in X$, and $C \subset \atoms$. Then $C$ is an $\sb$-support of $x$ iff it is a $\perm$-support of $x$. \stoplemma +\startremark +It is not true that any $\perm$-support is an $\sb$-support. +The condition that $X$ is nominal, in the above lemma, is crucial. +Let $X = \atoms + 1$ and define the following $\sb$-action: $m \cdot a = m(a)$ if $m$ is injective, $m \cdot a = \ast$ if $m$ is non-injective, and $m \cdot \ast = \ast$. +This is a well-defined $\sb$-set, but is \emph{not nominal}. +Now consider $U(X)$, this is the $\perm$-set $\atoms + 1$ with the natural action, which is a \emph{nominal} $\perm$-set! +In particular, as a $\perm$-set each element has a finite support, but as a $\sb$-set the supports are infinite. + +This counterexample is similar to the exploding nominal sets of \citet[Gabbay07], but even worse behaved. +We like to call them \emph{nuclear sets}, since an element will collapse when hit by a non-injective map, no matter how far away the non-injectivity occurs. +\stopremark + For $M \in \{\sb, \perm\}$, any element $x \in X$ of a nominal $M$-set $X$ has a least finite support (w.r.t. set inclusion). We denote the least finite support of an element $x \in X$ by $\supp(x)$. Note that by \in{Lemma}[lem:GM-support], the set $\supp(x)$ is independent of whether a nominal $\sb$-set $X$ is viewed as an $\sb$-set or a $\perm$-set. @@ -256,7 +268,6 @@ $U$ maps the exponent object in $\sbnom$ to the right adjoint $\wandto$ of the s \stopitemize Together, these results form the categorical infrastructure to relate nominal languages to separated languages and automata in \in{Section}[sec:automata]. -\todo{Reconsider moving some proofs from the appendix to the main text}. \startdefinition[reference=def:left-adjoint] Given a $\perm$-nominal set $X$, we define a nominal $\sb$-set $F(X)$ as follows. @@ -293,6 +304,45 @@ The construction $F$ in \in{Definition}[def:left-adjoint] extends to a functor defined on an equivariant map $f \colon X \to Y$ by $F(f)([m, x]) = [m, f(x)] \in F(Y)$. \stopproposition +\startproofnoqed +\startdescription{$F(X)$ is an $\sb$-set.} +We first check whether the $\sb$-action is well-defined. +Let $[m_1, x_1] = [m_2, x_2] \in F(X)$ and let $m \in \sb$. +By \in{Lemma}[lm:sim], there is some permutation $g$ such that $g x_1 = x_2$ +and $m_1|_C = m_2 g|_C$ for some support $C$ of $x_1$. +By post-composition with $m$ we get $m m_1|_C = m m_2 g|_C$, which means (again by the lemma) +that $[m m_1, x_1] = [m m_2, x_2]$. +Thus $m [m_1, x_1] = m [m_2, x_2]$, which concludes well-definedness. + +For associativity and unitality of the $\sb$-action, we simply note that it is +directly defined by left multiplication of $\sb$ which is associative and unital. +This concludes that $F(X)$ is an $\sb$-set. +\stopdescription +\startdescription{$F(X)$ is an $\sb$-nominal set.} +Given an element $[m, x] \in F(X)$ and a $\perm$-support $C$ of $x$, we will prove that $m \cdot C$ +is an $\sb$-support for $[m, x]$. +Suppose that we have $m_1, m_2 \in \sb$ such that $m_1|_{m\cdot C} = m_2|_{m\cdot C}$. +By pre-composition with $m$ we get $m_1 m|_C = m_2 m|_C$ and this leads us to conclude $[m_1 m, x] = [m_2 m, x]$. +So $m_1 [m,x] = m_2 [m, x]$ as required. +\stopdescription +\startdescription{Functoriality.} +Let $f \colon X \to Y$ be a $\perm$-equivariant map. +To see that $F(f)$ is well-defined consider $[m_1, x_1] = [m_2, x_2]$. +By \in{Lemma}[lm:sim], there is a permutation $g$ such that $g x_1 = x_2$ +and $m_1|_C = m_2 g|_C$ for some support $C$ of $x_1$. +Applying $F(f)$ gives on one hand $[m_1, f(x_1)]$ and +on the other hand $[m_2, f(x_2)] = [m_2, f(g x_1)] = [m_2, g f(x_1)] = [m_2 g, f(x_1)]$ +(we used equivariance in the second step). Since $m_1|_C = m_2 g|_C$ and $f$ preserves supports +we have $[m_2 g, f(x_1)] = [m_1, f(x_1)]$. + +For $\sb$-equivariance we consider both $n \cdot F(f)([m, x]) = n [m, f(x)] = [n m, f(x)]$ and $F(f)(n \cdot [m, x]) = F(f)([nm, x]) = [nm, f(x)]$. +This shows that $n F(f)([m, x]) = F(f)(n [m, x])$ and concludes that we have a map $F(f) \colon F(X) \to F(Y)$. + +The fact that $F$ preserves the identity function and composition follows from the definition directly. +\QED +\stopdescription +\stopproofnoqed + \starttheorem[reference=thm:adjunction] The functor $F$ is left adjoint to $U$: \placefigure[force, none]{}{ @@ -334,19 +384,19 @@ The functor $F$ not only preserves coproducts, being a left adjoint, but it also maps the separated product to products: \starttheorem[reference=thm:monoidal] -The functor $F$ is strong monoidal, from the monoidal category $(\permset, \sepprod, 1)$ to $(\sbset, \times, 1)$. +The functor $F$ is strong monoidal, from the monoidal category\break $(\permset, \sepprod, 1)$ to $(\sbset, \times, 1)$. In particular, the map $p$ given by \startformula p = \langle F(\pi_1), F(\pi_2) \rangle \colon F(X \sepprod Y) \to F(X) \times F(Y) \stopformula is an isomorphism, natural in $X$ and $Y$. \stoptheorem -\startproof +\startproofnoqed We prove that $p$ is an isomorphism. It suffices to show that $p$ is injective and surjective. Note that $p([m, (x, y)]) = ([m, x], [m, y])$. -\startdescription[title={Surjectivity}] +\startdescription[title={Surjectivity.}] Let $([m_1, x], [m_2, y])$ be an element of $F(X) \times F(Y)$. We take an element $y' \in Y$ such that $y' \separated \supp(x)$ and $y' = gy$ for some $g \in \perm$. Now we have an element $(x, y') \in X \sepprod Y$. @@ -372,7 +422,7 @@ and since $[mg, y] = [m, g y] = [m, y']$ we are done. Hence $p([m, (x, y')]) = ([m, x], [m, y']) = ([m_1, x], [m_2, y])$, so $p$ is surjective. \stopdescription -\startdescription[title={Injectivity}] +\startdescription[title={Injectivity.}] Let $[m_1, (x_1, y_1)]$ and $[m_2, (x_2, y_2)]$ be two elements. Suppose that they are mapped to the same element, i.e., $[m_1, x_1] = [m_2, x_2]$ and $[m_1, y_1] = [m_2, y_2]$. Then there are permutations $g_x, g_y$ such that $x_2 = g_x x_1$ and $y_2 = g_y y_1$. @@ -394,12 +444,13 @@ We also obtain $m_1|_{C \cup D} = m_2 g|_{C \cup D}$. This proves that $[m_1, (x_1, y_1)] = [m_2, (x_2, y_2)]$, and so the map $p$ is injective. \stopdescription -\startdescription[title={Unit and coherence}] +\startdescription[title={Unit and coherence.}] To show that $F$ preserves the unit, we note that $[m, 1] = [m', 1]$ for every $m, m' \in \sb$, as the empty set supports $1$ and so $m|_{\emptyset} = m'|_{\emptyset}$ vacuously holds. We conclude $F(1)$ is a singleton. By the definition $p([m, (x, y)])=([m, x], [m, y])$, one can check the coherence axioms elementary. +\QED \stopdescription -\stopproof +\stopproofnoqed Since $F$ also preserves coproducts (being a left adjoint), we obtain that $F$ maps the set of separated words to the set of all words. \startcorollary[reference=cor:sep-words] @@ -418,7 +469,18 @@ Concretely, for $X \in \permnom$ and $Y \in \sbnom$: \stopitemize \stoplemma -To appreciate the above result, we give a concrete characterisation of one-dimensional nominal sets: +The first item follows easily by showing that $\eta$ is surjective. +However, the second is non-trivial and uses the following technical property of $\leq 1$-dimensional $\sb$-sets. + +\startlemma[reference=lem:1dim-sbnom] +Let $Y$ be a nominal $\sb$-set. +If an element $y \in Y$ is supported by a singleton set (or even the empty set), then +\startformula +\{ m y \mid m \in \sb \} = \{ g y \mid g \in \perm \} . +\stopformula +\stoplemma + +To appreciate the above results, we give a concrete characterisation of one-dimensional nominal sets: \startlemma[reference=lm:char-dim-one] Let $X$ be a nominal $M$-set, for $M \in \{\sb,\perm\}$. Then $\dim(X) \leq 1$ iff there exist (discrete) sets $Y$ and $I$ such that $X \cong Y + \coprod_{I} \atoms$. @@ -445,11 +507,27 @@ We define a $\perm$-equivariant map $\phi$ as follows: \startformula \phi \colon (X \wandto U(Y)) \to U(F(X) \sbto Y) \stopformula -is defined by transposing (first by Currying, then by the adjunction) the following composition +is defined by using the composition \startformula -F(X \wandto U(Y)) \times F(X) \xrightarrow{p^{-1}} F((X \wandto U(Y)) \sepprod X) \xrightarrow{F(\ev)} FU(Y) \xrightarrow{\epsilon} Y +F(X \wandto U(Y)) \times F(X) \tot{p^{-1}} F((X \wandto U(Y)) \sepprod X) \tot{F(\ev)} FU(Y) \tot{\epsilon} Y, \stopformula where $p^{-1}$ is from \in{Theorem}[thm:monoidal] and $\ev$ is the evaluation map of the exponent $\wandto$. +By Currying and the adjunction we arrive at $\phi$: + +\placefigure[force,none]{}{ +\hbox{\starttikzpicture[node distance=16pt] +\node (1) {$F(X \wandto U(Y)) \times F(X) \to Y$}; +\node [below of=1] (2) {$F(X \wandto U(Y)) \to (F(X) \sbto Y)$}; +\node [below of=2] (3) {$(X \wandto U(Y)) \to U(F(X) \sbto Y)$}; +\node [fit=(1) (2), inner sep=1pt] (12) {}; +\node [fit=(2) (3), inner sep=1pt] (23) {}; + +\node [anchor=west] at (12.east) {by Currying}; +\node [anchor=west] at (23.east) {by \in{Theorem}[thm:adjunction]}; + +\draw (12.west) -- (12.east); +\draw (23.west) -- (23.east); +\stoptikzpicture}} \stopdefinition With this map we can prove a generalisation of \in{Theorem}[thm:adjunction]. @@ -460,6 +538,39 @@ Second, it extends the correspondence to all finitely supported maps and not jus \starttheorem[reference=thm:exponent-separated] The sets $X \wandto U(Y)$ and $U(F(X) \sbto Y)$ are naturally isomorphic via $\phi$ as nominal $\perm$-sets. \stoptheorem +\startproof +We define some additional maps in order to construct the inverse of $\phi$. +First, from \in{Theorem}[thm:adjunction] we get the following isomorphism: +\startformula +q \colon U(X \times Y) \tot{=} U(X) \times U(Y) +\stopformula +Second, with this map and Currying, we obtain the following two natural maps: + +\placefigure[force,none]{}{ +\hbox{\starttikzpicture[node distance=16pt] +\node (1) {$U(F(X) \sbto Y) \times UF(X) \tot{q^{-1}} U((F(X) \sbto Y) \times F(X)) \tot{U(\ev)} U(Y)$}; +\node [below of=1] (2) {$\alpha \colon U(F(X) \sbto Y) \to (UF(X) \permto U(Y))$}; +\node [fit=(1) (2), inner sep=1pt] (12) {}; + +\node [anchor=west] at (12.east) {by Currying}; + +\draw (12.west) -- (12.east); +\stoptikzpicture}} + +\placefigure[force,none]{}{ +\hbox{\starttikzpicture[node distance=16pt] +\node (1) {$(UF(X) \permto U(Y)) \times X \tot{\id {\times} \eta} (UF(X) \permto U(Y)) \times UF(X) \tot{\ev} U(Y)$}; +\node [below of=1] (2) {$\beta \colon (UF(X) \permto U(Y)) \to (X \permto U(Y))$}; +\node [fit=(1) (2), inner sep=1pt] (12) {}; + +\node [anchor=west] at (12.east) {by Currying}; + +\draw (12.west) -- (12.east); +\stoptikzpicture}} + +A calculation shows that $r \circ \beta \circ \alpha$ is the inverse of $\phi$. +\footnote{We use $r_{X, U(Y)} \colon (X \permto U(Y)) \to (X \wandto U(Y))$ here.} +\stopproof Note that this theorem gives an alternative characterisation of the magic wand in terms of the exponent in $\sbnom$, if the codomain is $U(Y)$. Moreover, for a $1$-dimensional object $X$ in $\sbnom$, we obtain the following special case of the theorem (using the co-unit isomorphism from \in{Lemma}[lem:1dim-iso]): @@ -642,7 +753,24 @@ Then $\overline{S}(a_1a_2a_3\cdots a_n) = m \cdot S(b_1 b_2 b_3 \cdots b_n)$. \stoptheorem \startproof There is the following chain of one-to-one correspondences, from the results of the previous section: -\todo{Chain of one-to-one correspondences}. +\placefigure[force,none]{}{ +\hbox{\starttikzpicture[node distance=16pt] +\node (1) {$(U\Sigma)^{(*)} \rightarrow UO$}; +\node [below of=1] (2) {$F(U\Sigma)^{(*)} \rightarrow O$}; +\node [below of=2] (3) {$(FU\Sigma)^* \rightarrow O$}; +\node [below of=3] (4) {$\Sigma^* \rightarrow O$}; +\node [fit=(1) (2), inner sep=1pt] (12) {}; +\node [fit=(2) (3), inner sep=1pt] (23) {}; +\node [fit=(3) (4), inner sep=1pt] (34) {}; + +\node [anchor=west] at (12.east) {by {\in{Theorem}[thm:adjunction]} }; +\node [anchor=west] at (23.east) {by {\in{Corollary}[cor:sep-words]} }; +\node [anchor=west] at (34.east) {by {\in{Lemma}[lem:1dim-iso]} }; + +\draw (12.west) -- (12.east); +\draw (23.west) -- (23.east); +\draw (34.west) -- (34.east); +\stoptikzpicture}} \stopproof Thus, every separated automaton over $U(\Sigma), U(O)$ gives rise to an $\sb$-language $\overline{S}$, corresponding to the language $S$ accepted by the automaton. @@ -653,7 +781,10 @@ Hence, in such a case, the restricted separated automaton suffices to describe t \startdefinition[reference=def:restr-aut] Let $i \colon Q \sepprod U(\Sigma) \to Q \times U(\Sigma)$ be the natural inclusion map. -A nominal automaton $\mathcal{A} = (Q, \delta, o, q_0)$ induces a separated automaton $\mathcal{A}_*$, by setting $\mathcal{A}_* = (Q, \delta \circ i, o, q_0)$. +A nominal automaton $\mathcal{A} = (Q, \delta, o, q_0)$ induces a separated automaton $\mathcal{A}_*$, by setting +\startformula +\mathcal{A}_* = (Q, \delta \circ i, o, q_0). +\stopformula \stopdefinition \starttheorem[reference=thm:separated-sb-lang]