diff --git a/draft/draft.tex b/draft/draft.tex index f67805b..33f7263 100644 --- a/draft/draft.tex +++ b/draft/draft.tex @@ -2334,7 +2334,26 @@ Barr relator is a generalization of the Egli-Milner relator, where the functor i \begin{definition}[Cogood Order Structure]\label{def:cogood-ord} A \emph{cogood order structure} on a functor $F$ is a preorder $\appr$ on each Hom-set of the form $\Hom(X,FY)$ that is a natural order structure, and if $h\c X\to FZ$, $k\c X\to FY$, $g\c Y\to Z$, $Fg\comp k\appr h $ in $\Hom(X,FZ)$, then there is $k'\c X\to FY$ such that $k\appr k'$ in $\Hom(X,FY)$ and $h=Fg\comp k'$. \end{definition} -\begin{lemma} +\begin{lemma}\label{lem:good} + Assuming that a a functor $F$ has an order structure $\appr$ that is good, then for every $f\in\Hom(X,FY)$ we have: + \begin{enumerate}[label=(\Roman*), ref=(\Roman*)] + \item $Ff\comp\sappr\quad=\quad\sappr\comp Ff$ + \item $(Ff)^\op\comp\appr\quad=\quad\appr\comp (Ff)^\op$ + \end{enumerate} +\end{lemma} +\begin{proof} + $(I)$ Assuming $t\mathrel{Ff\comp\sappr} x$, there exists $s$ such that $t\sappr s$ and $Ff(s)=x$. Since $\appr$ is good, and thus natural, by~\autoref{def:nat-ord}, from $s\appr t$ we get $x\appr Ff(t)$ that is $t\mathrel{\sappr\comp Ff} x$. + + Assuming $t\mathrel{\sappr\comp Ff} x$, there exists $y$ such that $Ff(t)=y$ and $y\sappr x$. By~\autoref{def:good-ord} since $Ff(t)\sappr x$ there exists $s$ that $t\sappr s$ and $Ff(s)=x$ that is $t\mathrel{Ff\comp\appr}s$. + + $(II)$ Basically, by definition of $\op$ and relation composition we have + \begin{gather*} + (Ff\comp\appr)^\op=\sappr\comp(Ff)^\op,\\ + (\appr\comp Ff)^\op=(Ff)^\op\comp\sappr. + \end{gather*} + So it follows directly from applying $\op$ on both sides of $(I)$.\qed +\end{proof} +\begin{lemma}\label{lem:cogood} Assuming that a a functor $F$ has an order structure $\appr$ that is cogood, then for every $f\in\Hom(X,FY)$ we have: \begin{enumerate}[label=(\Roman*), ref=(\Roman*)] \item $Ff\comp\appr\quad=\quad\appr\comp Ff$ @@ -2342,11 +2361,16 @@ Barr relator is a generalization of the Egli-Milner relator, where the functor i \end{enumerate} \end{lemma} \begin{proof} - $(I)$Assuming $t\mathrel{Ff\comp\appr} x$, there exists $s$ such that $t\appr s$ and $Ff(s)=x$. Since $\appr$ is good, and thus natural, by~\autoref{def:nat-ord}, from $t\appr s$ we get $Ff(t)\appr x$ that is $t\mathrel{\appr\comp Ff} x$. + $(I)$ Assuming $t\mathrel{Ff\comp\appr} x$, there exists $s$ such that $t\appr s$ and $Ff(s)=x$. Since $\appr$ is cogood, and thus natural, by~\autoref{def:nat-ord}, from $t\appr s$ we get $Ff(t)\appr x$ that is $t\mathrel{\appr\comp Ff} x$. Assuming $t\mathrel{\appr\comp Ff} x$, there exists $y$ such that $Ff(t)=y$ and $y\appr x$. By~\autoref{def:cogood-ord} since $Ff(t)\appr x$ there exists $s$ that $t\appr s$ and $Ff(s)=x$ that is $t\mathrel{Ff\comp\appr}s$. - $(II)$\todo{Finish.} + $(II)$ Basically, by definition of $\op$ and relation composition we have + \begin{gather*} + (Ff\comp\appr)^\op=\sappr\comp(Ff)^\op,\\ + (\appr\comp Ff)^\op=(Ff)^\op\comp\sappr. + \end{gather*} + So it follows directly from applying $\op$ on both sides of $(I)$.\qed \end{proof} \begin{prop} For a span $(\pi_1\c A\to X,\pi_2\c A\to Y)$, assuming that $F$ has a cogood order structure $\appr$, the following propositions hold: @@ -2375,34 +2399,21 @@ Barr relator is a generalization of the Egli-Milner relator, where the functor i Since $\appr$ is a cogood order structure by~\autoref{def:cogood-ord} there exists a $w$ such that $z\appr w$ and $F\pi_j(w)=y$. So, we have $w \mathrel{(F\pi_j)} y$, $z\mathrel{\appr} w$, and $z\mathrel{(F\pi_i)} x$ that gives $x \mathrel{F\pi_j\comp\appr\comp(F\pi_i)^\op} y$.\qed \end{proof} \begin{prop} - For a span $(\pi_1\c A\to X,\pi_2\c A\to Y)$, assuming that $F$ has a cogood order structure $\appr$, the following propositions hold: + For a span $(\pi_1\c A\to X,\pi_2\c A\to Y)$, assuming that $F$ has an order structure $\appr$, the following propositions hold: \begin{enumerate} - \item $F\pi_2\comp\sappr\comp(F\pi_1)^\op\quad=\quad F\pi_2\comp(F\pi_1)^\op\comp\appr$ - \item $F\pi_1\comp\sappr\comp(F\pi_2)^\op\quad=\quad F\pi_1\comp(F\pi_2)^\op\comp\appr$ + \item If $\appr$ is good, we have $F\pi_2\comp\appr\comp(F\pi_1)^\op\quad=\quad F\pi_2\comp(F\pi_1)^\op\comp\appr$ + \item If $\appr$ is cogood, we have $F\pi_2\comp\appr\comp(F\pi_1)^\op\quad=\quad \appr\comp F\pi_2\comp(F\pi_1)^\op$ + \item If $\appr$ is both good and cogood, all the following are equal: + \begin{itemize} + \item $F\pi_2\comp\appr\comp(F\pi_1)^\op\quad$ + \item $F\pi_2\comp(F\pi_1)^\op\comp\appr$ + \item $\appr\comp F\pi_2\comp(F\pi_1)^\op$ + \item $\appr\comp F\pi_2\comp(F\pi_1)^\op\comp\appr$ + \end{itemize} \end{enumerate} \end{prop} \begin{proof} - \todo{Finish.} -\end{proof} -\begin{prop} - For a span $(\pi_1\c A\to X,\pi_2\c A\to Y)$, assuming that $F$ has a cogood order structure $\appr$, the following propositions hold: - \begin{enumerate} - \item $F\pi_2\comp\appr\comp\sappr\comp(F\pi_1)^\op\quad=\quad \appr\comp F\pi_2\comp(F\pi_1)^\op\comp\appr$ - \item $F\pi_1\comp\appr\comp\sappr\comp(F\pi_2)^\op\quad=\quad \appr\comp F\pi_1\comp(F\pi_2)^\op\comp\appr$ - \end{enumerate} -\end{prop} -\begin{proof} - \todo{Finish.} -\end{proof} -\begin{prop} - For a span $(\pi_1\c A\to X,\pi_2\c A\to Y)$, assuming that $F$ has a good order structure $\appr$, the following propositions hold: - \begin{enumerate} - \item $F\pi_2\comp\sappr\comp(F\pi_1)^\op\quad=\quad\appr\comp F\pi_2\comp(F\pi_1)^\op$ - \item $F\pi_1\comp\sappr\comp(F\pi_2)^\op\quad=\quad\appr\comp F\pi_1\comp(F\pi_2)^\op$ - \end{enumerate} -\end{prop} -\begin{proof} - \todo{Finish.} + They all follow in an obvious way from~\autoref{lem:good} and~\autoref{lem:cogood}. The last one needs $\appr\comp\appr=\appr$ that comes from transitivity of $\appr$. \qed \end{proof} \begin{prop} Assuming that $\relar$ is a difunctionally functorial relator, then the symmetrization of the relator that takes $r\c X\rto Y$ to $\relar r\comp\appr$ is a sound relator.