A geometric morphism $f : E \to F$ between toposes is a functor of the underlying categories that is consistent with the interpretation of $E$ and $F$ as generalized topological spaces.
If $F = Set = Sh(*)$ is the terminal sheaf topos, then $E \to Set$ is essential if $E$ is a locally connected topos. In general, $f$ being essential is a necessary (but not sufficient) condition to ensure that $f$ behaves like a map of topological spaces whose fibers are locally connected: that it is a locally connected geometric morphism.
Given a geometric morphism $(f^* \dashv f_*) : E \to F$, it is an essential geometric morphism if the inverse image functor $f^*$ has not only the right adjoint $f_*$, but also a left adjoint $f_!$:
A point of a topos $x : Set \to E$ which is given by an essential geometric morphism is called an essential point of $E$.
There are various further conditions that can be imposed on a geometric morphism:
If $f_!$ can be made into an $E$-indexed functor and $f^*$ satisfies some extra conditions, the geometric morphism $f$ is a locally connected geometric morphism (see there for details).
If $f_!$ preserves finite products then $f$ is called connected surjective.
If in addition to the above $f$ is a local geometric morphism in that there is a further functor $f^! : F \to E$ which is right adjoint $(f_* \dashv f^!)$ and full and faithful then the geometric morphism $f$ is called cohesive.
Since for a Grothendieck topos $E$ the inverse image $x^*$ of an essential point is of the form $Hom_E(P,-)$ where $P\ncong\emptyset$ is projective and connected, objects satisfying these three conditions are sometimes called essential objects (cf. Johnstone 1977, p.255).
The inverse image $f^\ast$ of an essential geometric morphism preserves small limits since it is a right adjoint. Hence, this provides a minimal requirement to satisfy for a general geometric morphism $f^\ast\dashv f_\ast$ in order to qualify for being essential. In case, the toposes involved are Grothendieck toposes this condition is not only necessary but sufficient.
Let $f^\ast\dashv f_\ast:\mathcal{E}\to\mathcal{F}$ a geometric morphism between Grothendieck toposes. Then $f$ is essential iff $f^\ast$ preserves small limits iff $f^\ast$ preserves small products.
Grothendieck toposes are locally presentable and $f^\ast$ has rank i.e. it preserves $\alpha$-filtered colimits for some regular cardinal $\alpha$ since it is a left adjoint. But by (Borceux vol. 2 Thm.5.5.7, p.275) a functor between two locally presentable categories has a left adjoint precisely if it has rank and preserves small limits.
Since limits can be constructed from products and equalizers and $f^\ast$ preserves the latter, it preserves small limits precisely when it preserves small products.
For $C$ and $D$ small categories write $[C,Set]$ and $[D,Set]$ for the corresponding copresheaf toposes. (If we think of the opposite categories $C^{op}$ and $D^{op}$ as sites equipped with the trivial coverage, then these are the corresponding sheaf toposes.)
This construction extends to a 2-functor
from the 2-category Cat${}_{small}$ with 2-morphisms reversed) to the sub-2-category of Topos on essential geometric morphisms, where a functor $f : C \to D$ is sent to the essential geometric morphism
where $Lan_f$ and $Ran_f$ denote the left and right Kan extension along $f$, respectively.
This 2-functor is a full and faithful 2-functor when restricted to Cauchy complete categories:
For all small categories $C,D$ we have an equivalence of categories
between the opposite category of the functor category between the Cauchy completions of $C$ and $D$ and the the category of essential geometric morphisms between the copresheaf toposes and geometric transformations between them.
In particular, since every poset – when regarded as a category – is Cauchy complete, we have
Sometimes it is useful to decompose this statement as follows.
There is a functor
which assigns to each poset a locale called its Alexandroff locale. By a theorem discussed there, a morphisms of locales $f : X \to Y$ is in the image of this functor precisely if its inverse image morphism $f^* Op(Y) \to Op(X)$ of frames has a left adjoint in the 2-category Locale.
Moreover, for any poset $P$ the sheaf topos over $Alex P$ is naturally equivalent to $[P,Set]$
With this, the fact that $[-,Set] : Poset \to Topos$ hits precisely the essential geometric morphisms follows with the basic fact about localic reflection, which says that $Sh : Locale \to Topos$ is a full and faithful 2-functor.
Let $f : E \to F$ be an essential geometric morphism.
For every $\phi : X \to f^* f_* A$ in $E$ the diagram
commutes, where the vertical morphisms are unit and counit, respectively, and where the bottom horizontal morphism is the adjunct of $\phi$ under the composite adjunction $(f^* f_! \dashv f^* f_*)$.
The morphism $\phi : X \to f^* f_* A$ is the component of a natural transformation
The composite $X \stackrel{\phi}{\to} f^* f_* A \to A$ is the component of this composed with the counit $f^* f_* \Rightarrow Id$.
We may insert the 2-identity given by the zig-zag law
Composing this with the counit $f^* f_* \Rightarrow Id$ produces the transformation whose component is manifestly the morphism $X \to f^* f_! X \to A$.
A logical functor $E\to F$ with a left adjoint has automatically also a right adjoint whence is the inverse image part of an essential geometric morphism $F\to E$.
A particularly important instance of this situation is the following:
For any morphism $f\colon A\to B$ in a topos $E$, the induced geometric morphism $f\colon E/A \to E/B$ of overcategory toposes is essential. Here, the logical functor is given by the pullback functor $f^*:E/B\to E/A$ of course.
In the special case $B = *$ the terminal object, the essential geometric morphism
is also called an etale geometric morphism.
Conversely, (almost) any essential geometric morphism $\pi$ whose inverse image $\pi^*$ is a logical functor has the form of an etale geometric morphism:
If the left adjoint $\pi_!$ of a logical functor $\pi^*:E\to F$ furthermore preserves equalizers, then the corresponding essential geometric morphism is up to equivalence an etale geometric morphism $\pi : F\simeq E/A \to E$ for an object $A$ in $E$ that is determined up to isomorphism.
For a proof see e.g. Johnstone (1977, p.37).
A locally connected topos $E$ is one where the global section geometric morphism $\Gamma : E \to Set$ is essential.
In this case, the functor $\Gamma_! = \Pi_0 : E \to Set$ sends each object to its set of connected components. More on this situation is at homotopy groups in an (∞,1)-topos.
Note, though that if $p\colon E\to S$ is an arbitrary geometric morphism through which we regard $E$ as an $S$-topos, i.e. a topos “in the world of $S$,” the condition for $E$ to be locally connected as an $S$-topos is not just that $p$ is essential, but that the left adjoint $p_!$ can be made into an $S$-indexed functor (which is automatically true for $p^*$ and $p_*$). This is automatically the case for $Set$-toposes (at least, when our foundation is material set theory—and if our foundation is structural set theory, then our large categories and functors all need to be assumed to be $Set$-indexed anyway). For more see locally connected geometric morphism.
The tiny objects of a presheaf topos $[C,Set]$ are precisely the essential points $Set \to [C,Set]$. See tiny object for details.
Like many other things, it all started as an exercise in
Speaking of exercises, consider the results of Roos on essential points reported in exercise 7.3 of
The case of sheaves valued in FinSet is considered in
The standard reference for essential localizations ^{1}, aka levels, is
For more on this see also
The definition of essential geometric morphisms appears before Lemma A.4.1.5 in
Connected surjective and local geometric morphisms are discussed in
Further refinements are in