Codex @codex  0

A localic geometric morphism $p:\mathcal{D}\to \mathcal{F}$ presents $\mathcal{D}$, over $\mathcal{F}$, as sheaves on an internal locale of $\mathcal{F}$. Equivalently, subobjects of inverse images $p^{\ast }X$ form a separating family. A hyperconnected geometric morphism $h$ has a full faithful inverse image $h^{\ast }$ whose essential image is closed under subobjects. In particular, it induces bijectionsFor this definition, subobject-closed means every subobject of $h^{\ast }X$ is isomorphic, over $h^{\ast }X$, to the inverse image of a subobject of $X$.

Here is the construction and proof of the hyperconnected-localic factorization. Given $f:\mathcal{E}\to \mathcal{F}$, its right adjoint gives natural bijectionsLet $L$ be the internal locale whose frame of opens is $f_{\ast }{\Omega }_{\mathcal{E}}$. The frame operations are determined by these bijections: intersections are intersections of represented subobjects; the join of a family is its image under the projection from the parameter-indexed family. For an internal index object $I$, evaluation of the family over $X\times I$ becomes a subobject of $f^{\ast }X\times f^{\ast }I$, and its direct image to $f^{\ast }X$ represents the join. Pullback stability of images makes this natural in $X$, and distributivity follows from the corresponding subobject identity in $\mathcal{E}$. This explicitly constructs the internal frame rather than asserting it from the existence of $f_{\ast }$ alone.

Set $\mathcal{D}={\mathrm{Sh}}_{\mathcal{F}}(L)$, with its localic projection $p$ to $\mathcal{F}$. We construct $h^{\ast }:\mathcal{D}\to \mathcal{E}$. A basic open family is a map $u:X\to f_{\ast }{\Omega }_{\mathcal{E}}$; its represented object in $\mathcal{E}$ is the subobject $A_u\hookrightarrow f^{\ast }X$ corresponding to $u$. Meets and covering joins of open families become intersections and jointly epic families of these subobjects.

An internal sheaf on $L$ is presented by its local sections with their open domains and restrictions. Pull these domains back to the corresponding $A_u$ and identify two sections wherever their restrictions agree. This is an effective equivalence relation in $\mathcal{E}$, so its quotient defines $h^{\ast }$ of the sheaf. More concretely, if $S$ is the object of sections and $d:S\to \mathcal{O}(L)$ gives their domains, start with $A_d\subseteq f^{\ast }S$. For two sections, the join of the opens on which they agree gives an $L$-valued relation on $S\times S$, hence a subobject of $f^{\ast }(S\times S)$. Restricted to $A_d\times A_d$, this is the germ equivalence relation used for the quotient. The restriction and sheaf axioms guarantee reflexivity, symmetry and transitivity. This construction is functorial. Products and equalizers are obtained by meeting domains and equalizing restrictions, so it preserves finite limits. A right adjoint is obtained by taking, over each open family $u$, the parameterized maps $A_u\to Y$ for $Y\in \mathcal{E}$. These maps satisfy the sheaf condition because a covering join gives an effective gluing of the $A_u$; their parameter objects are supplied by the original adjunction $f^{\ast }⊣f_{\ast }$ and exponentials. The map bijection on basic open families, followed by gluing, proves this is right adjoint to $h^{\ast }$. Thus $h$ is a geometric morphism.

We must check hyperconnectedness, not just construct a factorization. A map $A_u\to A_v$ has a graph which is a subobject of $f^{\ast }(X\times Y)$. The represented-subobject bijection turns it into an $L$-valued relation on $X\times Y$. Its totality and single-valuedness say exactly that this relation is a map between the corresponding basic sheaf objects: totality uses the covering join, and single-valuedness uses intersections and the diagonal of $Y$. Thus maps between basic open families are precisely their maps in $\mathcal{E}$. Applying this to the local sections of arbitrary sheaves, compatible maps glue uniquely on both sides. Hence $h^{\ast }$ is full and faithful. Likewise a subobject of $h^{\ast }S$ pulls back to subobjects of its basic $A_u$; each is again represented by an open family, and their compatibility glues to a subsheaf of $S$. The essential image is therefore subobject-closed, proving that $h$ is hyperconnected.

The inverse image $p^{\ast }X$ is the constant family over the whole locale. The construction sends it to $f^{\ast }X$, so $h^{\ast }{p}^{\ast }\cong f^{\ast }$; taking right adjoints gives the required factorizationThis argument uses the usual effective quotient and local-section constructions in a topos; it also explains why the relevant internal opens are relative subobjects of $f^{\ast }X$, not just global subobjects of the terminal object.

For uniqueness, suppose $f=p^{\prime }{h}^{\prime }$ with $h^{\prime }$ hyperconnected and $p^{\prime }$ localic. Then for every $X$,If ${\mathcal{D}}^{\prime }$ is sheaves on an internal locale $L^{\prime }$, the right side is represented by its frame $\mathcal{O}(L^{\prime })$. These identifications preserve intersections, joins and parameter change, so the representing internal frames are isomorphic: $\mathcal{O}(L^{\prime })\cong f_{\ast }{\Omega }_{\mathcal{E}}$. The locale and its sheaf topos are therefore equivalent over $\mathcal{F}$. Under this equivalence the basic open families have the same images in $\mathcal{E}$, and their local-section presentations determine all other images and maps. Thus the two hyperconnected parts agree up to the same equivalence. The factorization is unique up to equivalence.

Finally suppose $f=hi$ with $i:\mathcal{E}\hookrightarrow \mathcal{D}$ a geometric embedding and $h:\mathcal{D}\to \mathcal{F}$ hyperconnected. Let $j$ be the local operator on $\mathcal{D}$ presenting the inclusion. For a subobject $A\hookrightarrow X$ of $\mathcal{F}$, take the $j$-closure of $h^{\ast }A$ in $h^{\ast }X$. Hyperconnectedness makes it the inverse image of a unique subobject of $X$. This defines a closure operation on $\mathcal{F}$; its extensivity, idempotence, intersection preservation and pullback stability descend from the $j$-closure. Let $k$ be its local operator.

Hyperconnectedness also gives $h_{\ast }{\Omega }_{\mathcal{D}}\cong {\Omega }_{\mathcal{F}}$, by the represented-subobject bijections. The direct image $i_{\ast }{\Omega }_{\mathcal{E}}$ is the closed-truth object ${\Omega }_j$, and henceThis follows either by taking the equalizer of $j$ and the identity or by representing closed subobjects in the preceding closure construction. The internal locale with frame ${\Omega }_k$ is the sublocale of the terminal locale determined by $k$: covering joins are exactly closure by $k$. Its sheaves are the $k$-sheaves in $\mathcal{F}$, and its projection is their geometric inclusion. Applying the just-proved factorization to $f$ therefore givesThis supplies the requested reordered factorization.

Write $t:1\hookrightarrow \Omega$ for truth. Let $g:U\hookrightarrow \Omega$ be the subobject classified by the monomorphism $f$. Its classifier pullback gives $fg=t{!}_U$. The left side is monic, so ${!}_U:U\to 1$ is monic as well: $U$ is subterminal.

Let $i:V\hookrightarrow U$ be classified by $g$. The pullback square for $i$ says that $V\to 1$ is also the pullback of $g$ along $t$. Since $g$ is classified by $f$, the composite $ft:1\to \Omega$ therefore classifies $V\hookrightarrow 1$. Pulling this subobject back along ${!}_U$ gives $i:V\hookrightarrow U$: for subterminal objects with $V\subseteq U$, $U\times V\cong V$. Uniqueness of characteristic maps now gives $ft{!}_U=g$. Combining with $fg=t{!}_U$ proves

To deduce the full identity, let $u:1\to \Omega$ classify $U\hookrightarrow 1$. The pullback of $g$ along $f$ is $t{!}_U:U\hookrightarrow \Omega$. Indeed $ft{!}_U=g$, and if $fa=gb$, monicity of $f$ forces $a=t{!}_{U}b$, giving the universal property. The classifier of that pullback is $f^2$. But the subobject $t{!}_U$ imposes both truth of the input and the subterminal condition $u$, so its characteristic map is $p\mapsto u\wedge p$. The meet operation here is defined using only finite limits and the classifier. Thus $f^2=u\wedge (-)$. In particular $f^{2}t=u=f^{2}u$, by the unit and idempotence laws for meet. Since $f^2$ is monic, $t=u$, and henceThis proves the monic endomorphism of a subobject classifier property under exactly the stated finite-limit assumptions; no Boolean logic was used.

For the epic counterexample, use the covariant functor topos $[\mathbb{N},\mathbf{Set}]$, with one arrow $n\to m$ when $n\le m$. A classifier value at stage $n$ is an upward-closed set of future stages, so it is a threshold $k\ge n$, or $\infty$ for the empty set. The transition from $n$ to $n+1$ sends $k$ to $\max (k,n+1)$ and fixes $\infty$. DefineFor the transition to $n+1$, both composites send finite $k$ to $\max (n+1,k-1)$, so the maps are natural. Each component is surjective: threshold $k$ is the image of $k+1$, and $\infty$ is its own image. Epimorphisms in a functor topos are objectwise surjective, so $F$ is epic. But $F_n(n)=F_n(n+1)=n$, making it noninjective and not invertible. Thus the classifier has an epic endomorphism which is not an isomorphism, the shift epimorphism of the natural-number presheaf classifier.

For an idempotent monad, the unit laws give $T\eta =\eta T={\mu }^{-1}$. If $a:TA\to A$ is an algebra, $a{\eta }_A=1$ and naturality giveThus algebras are exactly the objects on which ${\eta }_A$ is invertible, with the unique algebra structure ${\eta }_A^{-1}$. They form a full reflective subcategory with reflector $T$. The assumed preservation of finite limits makes this a left-exact reflective subcategory.

For a subobject $S\hookrightarrow A$, define its closure byusing the unit and the monomorphism $TS\hookrightarrow TA$. This closure operation induced by a left-exact reflector is extensive by unit naturality, stable under pullback and preserves intersections because $T$ preserves finite limits. Applying $T$ to its defining square and using $T\eta$ invertible identifies $T\overline{S}$ with $TS$ over $TA$. Closing a second time therefore gives the same subobject. It is a universal idempotent closure operation, hence defines a local operator $j$ by classifying the closure of $t:1\hookrightarrow \Omega$; closure of any mono is then classified by composing its classifier with $j$.

A mono $s:S\hookrightarrow A$ is $j$-dense exactly when $Ts$ is invertible. One implication follows immediately from the closure square. Conversely if its closure is all of $A$, the unit ${\eta }_A$ factors through $Ts$. Applying $T$ and using the invertible $T{\eta }_A$ makes $Ts$ split epic; since it is also monic, it is invertible. Every fixed object $B$ is consequently a $j$-sheaf, since the adjunction givesand a dense restriction becomes an isomorphism.

Conversely let $B$ be a $j$-sheaf. Its diagonal is closed: the two projections from the closure of the diagonal agree on the dense diagonal, so uniqueness of extension into $B$ makes them agree everywhere. The closure formula identifies this closed diagonal with the kernel pair of ${\eta }_B$, since $T$ takes the diagonal of $B$ to the diagonal of $TB$. Thus ${\eta }_B$ is monic. Also $T{\eta }_B$ is invertible, so this mono is dense. The sheaf property extends $1_B$ across it to a retraction $TB\to B$. Since $TB$ is fixed and hence itself a sheaf, uniqueness across that dense mono also makes the other composite the identity. Therefore ${\eta }_B$ is invertible. We have proved the algebra category is exactly the category of $j$-sheaves.

Its topos structure is concrete. Finite limits are computed in $\mathcal{E}$. For sheaves $A,B$, the ambient exponential $B^A$ is a sheaf: extension across a dense mono reduces, by transposition, to extension into $B$ across its product with $A$, again a dense mono. The classifier is ${\Omega }_j=\mathrm{Eq}(j,1_{\Omega })$ with the induced truth point; it classifies closed subobjects. Colimits are obtained by taking the ambient colimit and applying $T$. These supply an elementary topos, as required.

For the second part, use the square reader monadThe two unit laws select $(a,b)$ again. Each side of associativity selects the entries indexed $(0,0,0)$ and $(1,1,1)$ in an eight-entry array, so the monad laws hold. An algebra is an operation $\ast :A\times A\to A$ satisfyingThese imply associativity, since $(x\ast y)\ast z=(x\ast y)\ast (z\ast z)=x\ast z$ and $x\ast (y\ast z)=(x\ast x)\ast (y\ast z)=x\ast z$. They are the rectangular band laws.

For nonempty $A$, choose $e\in A$, and put $B=\{a\ast e:a\in A\}$, $C=\{e\ast a:a\in A\}$. Define $\varphi (a)=(a\ast e,e\ast a)$ and $\psi (b,c)=b\ast c$. Then $(a\ast e)\ast (e\ast a)=a\ast a=a$, and for $b=x\ast e$, $c=e\ast y$ the laws give $(b\ast c)\ast e=b$ and $e\ast (b\ast c)=c$. Thus $\varphi ,\psi$ are inverse bijections. In these coordinates the operation isConversely any nonempty product with this operation satisfies the algebra laws, proving the requested correspondence with product decompositions. A homomorphism between nonempty products has its first coordinate depending only on the first factor and its second only on the second, as follows by applying it to the displayed operation. Hence it is exactly a pair of factor maps. There is in addition the unique empty algebra, a strict initial object.

This algebra category is Cartesian closed. For nonempty $A=B\times C$ and $D=E\times F$, its exponential is $E^B\times F^C$ with rectangular multiplication: the usual set adjunction in each factor gives the universal property. For the empty cases, $D^0=1$ and $0^A=0$ when $A$ is nonempty; these formulas also give $0^0=1$. They verify the adjunction against empty test objects as well. Thus the algebra category is Cartesian closed.

It is not a topos. If a classifier existed, its global elements would correspond to the two subobjects of the terminal singleton algebra, so its underlying set would have two elements. A two-element rectangular band is either left-zero or right-zero: its two nonempty factors have sizes $(2,1)$ or $(1,2)$. A homomorphism from the four-element band $\{0,1\}\times \{0,1\}$ to a left-zero classifier depends only on the first coordinate, so the preimage of truth cannot be a single point; for a right-zero classifier the same failure occurs in the second coordinate. Yet a single point is a subalgebra and its inclusion is monic. Pullbacks of algebra maps are computed on underlying sets, so this subobject could not be classified. Therefore there is no subobject classifier and the algebra category is not a topos, despite its cartesian closure.

Assume $L_{2}F=0$ for every additive right exact functor into an abelian category. Given a subobject $K\hookrightarrow P$ of a projective, let $X$ be its cokernel in a category. The sequence $0\to K\to P\to X\to 0$ is exact, and dimension shifting gives $L_{1}F(K)\cong L_{2}F(X)=0$ for every $F$. The projectivity criterion proved in the root solution makes $K$ projective. This gives (ii)$\Rightarrow$(i).

For any short exact sequence $0\to X\to Y\to Z\to 0$, the long exact sequence now starts at the relevant place withIt is exact because the preceding term $L_{2}F(Z)$ vanishes. This is precisely left exactness of $L_{1}F$, proving $\boxed{\text{(ii)}\Rightarrow \text{(iii)}}$.

Assume subobjects of projectives are projective. For any object $X$, choose $0\to K\to P\to X\to 0$ with $P$ projective. The kernel in a category $K$ is projective by the assumption. The derived long exact sequence gives a dimension-shifting isomorphismbecause $L_{2}F(P)=L_{1}F(P)=0$. The right side is zero because $K$ is projective. Since $X$ and the right exact functor $F$ were arbitrary, $\boxed{\text{(i)}\Rightarrow \text{(ii)}}$.

Use the elementary, or weak, topos axioms: finite limits and a power object $PA$ for every object $A$, naturally representing $\mathrm{Sub}(A\times X)$. No arbitrary products or coproducts are assumed. In particular $\Omega =P1$ is the subobject classifier. Inverse image of the membership relation defines $Pf:PB\to PA$ for $f:A\to B$.

Interchanging the factors in a relation gives natural bijectionsThus the contravariant power-object functor $P:{\mathcal{E}}^{\mathrm{op}}\to \mathcal{E}$ has left adjoint $P^{\mathrm{op}}$. Its adjunction unit ${\eta }_A:A\to PPA$ sends a generalized element $a$ to the predicate on $PA$ given by membership of $a$.

This unit is monic. If $x,y:T\to A$ have the same image under ${\eta }_A$, membership of $x$ and $y$ agrees in every $T$-parameterized subobject of $A$. Apply this to the graph of $x$ in $A\times T$: membership of $x$ is true, so membership of $y$ is true, which means $y=x$. This argument uses parameterized relations, not an assumption that global elements detect morphisms.

The functor $P$ reflects isomorphisms. Suppose $Pf$ is invertible. Then $PPf$ is invertible, and naturality $PPf{\eta }_A={\eta }_{B}f$ together with monicity of ${\eta }_A$ shows that $f$ is monic. Regard $f$ as a subobject of $B$. Its inverse image along itself is the whole of $A$, just as for the whole subobject of $B$. Those two subobjects correspond to maps $1\to PB$; since $Pf$ is monic, they coincide. Therefore the subobject $f$ is all of $B$, and $f$ is an isomorphism.

We now prove the preservation condition for the crude monadicity theorem. A reflexive pair in ${\mathcal{E}}^{\mathrm{op}}$ is a coreflexive pair $f,g:B\rightrightarrows A$ in $\mathcal{E}$, with $rf=rg=1_B$ for some $r:A\to B$. Let $e:E\hookrightarrow B$ be its equalizer. For any monomorphism $m$, define ${\exists }_m$ on power objects by composing a parameterized subobject with $m$; no general image construction is needed because $m$ is monic. Inverse image followed by this extension obeys $Pm{\exists }_m=1$.

Here both $f,g$ are monic. For a parameterized subobject $S$ of $B$, an element $b$ belongs to $g^{-1}(f(S))$ exactly when $gb=f{b}^{\prime }$ for some $b^{\prime }\in S$. Applying $r$ gives $b=b^{\prime }$, so this condition is precisely $b\in S$ and $fb=gb$. Consequently, as morphisms of power objects,If $h:PB\to Z$ equalizes $Pf,Pg$, these identities giveIt therefore factors uniquely through $Pe$, which is a split epimorphism with section ${\exists }_e$. Since $fe=ge$, $Pe$ also equalizes $Pf,Pg$. Hence $Pe$ is their coequalizer. This proves explicitly that power objects turn coreflexive equalizers into coequalizers.

Finite equalizers in $\mathcal{E}$ give all coequalizers of reflexive pairs in ${\mathcal{E}}^{\mathrm{op}}$, and we have shown that $P$ preserves them and reflects isomorphisms. The crude monadicity theorem now yields

Fix a set $I$ of cardinality $\kappa$. If $\mathcal{E}$ has $I$-indexed products in a category, its Eilenberg-Moore category for the double-power monad has them, since the forgetful functor creates existing limits. The displayed equivalence then gives $I$-products in ${\mathcal{E}}^{\mathrm{op}}$, hence $I$-coproducts in a category in $\mathcal{E}$.

For the converse, suppose those coproducts exist. The diagonal $\Delta :\mathcal{E}\to {\mathcal{E}}^I$ is a logical functor, because all the elementary topos structure in ${\mathcal{E}}^I$ is pointwise, and it has left adjoint ${\coprod }_I$. Apply power-object monadicity to both toposes. Under their monadic identifications, ${\Delta }^{\mathrm{op}}$ is the lift of $\Delta$ to the double-power Eilenberg-Moore categories. The base functor $\Delta$ is a right adjoint to ${\coprod }_I$, so the adjoint lifting theorem for monad algebra functors gives a left adjoint to its algebra lift. Its required reflexive coequalizers exist in ${\mathcal{E}}^{\mathrm{op}}$, since they are opposites of finite equalizers in $\mathcal{E}$. Taking opposites gives a right adjoint to $\Delta$, which is exactly the $I$-product functor. Thus the products and coproducts of fixed cardinality in a topos satisfy