%! TEX root = CT.tex % vim: tw=50 % 02/12/2024 09AM \begin{fcdefn}[Support, totally supported, capital] \glsnoundefn{supp}{support}{supports}% \glsnoundefn{siobj}{strict initial object}{strict initial objects}% \glsadjdefn{ws}{well-supported}{object}% \glsadjdefn{ts}{totally-supported}{\gls{cat}}% \glsadjdefn{ats}{almost totally-supported}{\gls{cat}}% \glsadjdefn{cap}{capital}{\gls{cat}}% \phantom{} \begin{cenum}[(a)] \item By the \emph{support} of an object $A$ in a \gls{regc} \gls{cat}, we mean the \gls{image} of $A \to 1$. We say $A$ is \emph{well-supported} if $A \to 1$ is a \gls{cover}. \item We say a \gls{regc} \gls{cat} $\mathcal{C}$ is \emph{totally supported} if every object is well-supported. We say $\mathcal{C}$ is \emph{almost totally supported} if every object is either well-supported or a strict \gls{iobj}, where we cann an object $0$ \emph{strict} if every $A \to 0$ is an isomorphism. (Given finite \glspl{lim}, a strict object is \gls{initial} since for any $A$ there exists $0 \stackrel{\pi^{-1}}{\to} 0 \times A \stackrel{\pi_2}{\to} A$, and the \gls{equaler} of any pair $0 \twofuncs{}{} A$ is $a$). \item We say a \gls{regc} \gls{cat} $\mathcal{C}$ is \emph{capital} if its \gls{tobj} $1$ is a \gls{det}, i.e. $\mathcal{C}(1, \blank)$ \gls{reflects} isomorphisms. \end{cenum} \end{fcdefn} \begin{example*} $\Gp$ and $\AbGp$ are \gls{ts} since their \glspl{tobj} are \gls{initial}. $\Set$ is \gls{ats} and \gls{cap}. Note that \gls{cap} implies \gls{ats} since if $A$ isn't \gls{ws} there are no morphisms $1 \to A$. \end{example*} \glsadjdefn{cp}{cover-projective}{object}% A \gls{reple} \gls{func} $\mathcal{C}(A, \blank)$ always \gls{preses} \glspl{lim}, so it's a \gls{reg} \gls{func} if and only if $A$ is cover-projective (c.f. \cref{defn:2.10}). \begin{fclemma}[] \label{lemma:7.12} % Lemma 7.12 Assuming: - $\mathcal{C}$ a \gls{locsm} \gls{cap} \gls{regc} \gls{cat} Then: $1$ is \gls{cp}. \end{fclemma} \begin{proof} Since \glspl{cover} are stable under \gls{pullb}, we need to show that every $A \covers 1$ is \gls{splitme} \gls{epic}. If $A \cong 1$, nothing to prove. If not, the projections $A \times A \twofuncs{}{} A$ aren't equal (since their \gls{coequaler} is $A \covers 1$, by \cref{prop:7.5}). So there exists $1 \to A \times A$ not factoring through their \gls{equaler}, so there exists $1 \to A \times A \to A$. \end{proof} \glssymboldefn{ws}% If $\mathcal{C}$ is \gls{regc}, the \gls{full} sub\gls{cat} $\mathcal{C}_{\text{ws}}$ of \gls{ws} objects is closed under finite \glspl{prod} since \begin{picmath} \begin{tikzcd} A \times B \ar[r] \ar[d] & A \ar[d, {-Triangle[open]}] \\ B \ar[r, {-Triangle[open]}] & 1 \end{tikzcd} \end{picmath} is a \gls{pullb}, and under \glspl{pullb} of \glspl{cover} since if $A \covers B$ then $A$ and $B$ have the same \gls{supp}. \glssymboldefn{tv}% We write $\mathcal{C}_{\text{tv}}$ for the \gls{cat} obtained from $\mathcal{C}_\ws$ by adjoining a \gls{siobj} $0$: this is \gls{regc} and \gls{ats} and the \gls{func} $\mathcal{C} \to \mathcal{C}_{\text{tv}}$ sending all non-\gls{ws} objects to $0$ is \gls{reg} (c.f. Exercise 5.19). \begin{fclemma}[] \label{lemma:7.13} % Lemma 7.13 Assuming: - $\mathcal{C}$ a \gls{small} \gls{ats} \gls{regc} \gls{cat} Then: there exists an isomorphism-\glsref[reflects]{reflecting} \gls{reg} \gls{func} $I : \mathcal{C} \to \mathcal{C}'$, where $\mathcal{C}'$ is also \gls{small} and \gls{ats}, such that for every \gls{ws} $A \in \ob \mathcal{C}$ there exists a morphism $1 \to IA$ in $\mathcal{C}'$ not factoring through $I(m)$ for any proper \gls{subobj} $m : A' \monic A$ in $\mathcal{C}$. \end{fclemma} \begin{proof} Recall from Exercise 7.17: $\mathcal{C}$ \gls{regc} implies $\mathcal{C} / A$ \gls{regc} for any $A$, and for any $f : A \to B$ in $\mathcal{C}$ \gls{pullb} along $f$ defines a \gls{reg} \gls{func} $f^* : \mathcal{C} / B \to \mathcal{C} / A$, which has a \gls{ladj} $\Sigma_f : \mathcal{C} / A \to \mathcal{C} / B$ sending $g : C \to A$ to $fg$. And $f^*$ \gls{reflects} isomorphisms if and only if $f$ is a \gls{cover}. We'll define $\mathcal{C}'$ as $(\hat{\mathcal{C}})_\tv$ where $\hat{\mathcal{C}}$ is easier to describe. To satisfy the desired conclusion for a single \gls{ws} object $A$, enough to take $(!)_A^* : \mathcal{C} \cong \mathcal{C} / 1 \to \mathcal{C} / A$, since $(!_A)^* A = (A \times A \stackrel{\pi_2}{\to} A)$ acquires a point $\Delta : (A \stackrel{1}{\to} A) \to (A \times A \to A)$ not factoring through $(A' \times A \to A)$ for any proper $A \monic A$. More generally, for any finite list $A_1, \ldots, A_n$ of \gls{ws} objects, we can take $\mathcal{C} / \prod_{i = 1}^{n} A_i$. We define a \emph{base} to be a finite list $\vec{A} = (A_1, \ldots, A_n)$ of distinct \gls{ws} objecs of $\mathcal{C}$. We preorder the set $\mathcal{B}$ of bases by $\vec{A} \le \vec{B}$ if $\vec{B}$ contains all the members of $\vec{A}$. We write $\prod \vec{A}$ for the product $\prod_{i = 1}^{n} A_i$ and if $\vec{A} \le \vec{B}$ we write $\pi_{\vec{B}, \vec{A}}$ for the \gls{prod} projection $\prod \vec{B} \to \prod \vec{A}$. This makes $\vec{A} \mapsto \prod \vec{A}$ into a \gls{func} $\mathcal{B}^\op \to \mathcal{C}$. Hence the assignment $\vec{A} \to \mathcal{C} / \prod \vec{A}$, $\pi_{\vec{B}, \vec{A}} \mapsto \pi_{\vec{B}, \vec{A}}^*$ is `almost' a \gls{func} $\mathcal{B} \to \Cat$. We now define $\hat{\mathcal{C}}$: its objects are pairs $(\vec{B}, f)$ where $\vec{B}$ is a base and $f : A \to \prod \vec{B}$ is an object of $\mathcal{C} / \prod \vec{B}$. Morphisms $(\vec{B}, f) \to (\vec{B}', f')$ are \gls{reped} by pairs $(\vec{C}, g)$ where $\vec{C}$ is a base containing $\vec{B}$ and $\vec{B}'$ and $g : \pi^* f \to \pi'^* f'$ in $\mathcal{C} / \prod \vec{C}$, subject to the relation which identifies $(\vec{C}, g)$ with $(\vec{C}', g')$ if $\vec{\mathcal{C}} \le \vec{\mathcal{C}}'$ and the \gls{pullb} of $g$ to $\mathcal{C} / \prod \vec{\mathcal{C}}$ is isomorphic to $g'$. Clearly, each $\mathcal{C} / \prod \vec{B}$ sits inside $\hat{\mathcal{C}}$ as a non-\gls{full} sub\gls{cat}; so in particular $\mathcal{C} \cong \mathcal{C} / \prod []$ is a sub\gls{cat} of $\hat{\mathcal{C}}$, $\hat{\mathcal{C}}$ is \gls{regc}, and the inclusions $\mathcal{C} / \prod \vec{B} \to \hat{\mathcal{C}}$ are isomorphism-\glsref[reflects]{reflecting} \gls{reg} \glspl{func}. Given a finite \gls{diag} in $\hat{\mathcal{C}}$, we can choose $\vec{B}$ such that all edges of the \gls{diag} appear as morphisms in $\mathcal{C} / \prod \vec{B}$, and take the \gls{lim} there, and this is a \gls{lim} in $\hat{\mathcal{C}}$. Similarly for \glspl{image}. Also, if a morphism $f$ becomes an isomorphism in $\hat{\mathcal{C}}$, its inverse must live $\mathcal{C} / \prod \vec{B}$ for some $\vec{B}$, hence $f$ is an isomorphism $\mathcal{C} / \prod \vec{B}$. We define $\mathcal{C}' = (\hat{\mathcal{C}})_\tv$: the induced \gls{func} $\mathcal{C} \to \hat{\mathcal{C}} \to \mathcal{C}'$ is still isomorphism \glsref[reflects]{reflecting} since $\mathcal{C}$ is \gls{ats}. \end{proof}