A Conduché functor, also called a Conduché fibration or an exponentiable functor, is a functor which is an exponentiable morphism in Cat. (In accordance with Baez's law, the notion was actually defined in Giraud 64 before Conduché.) This turns out to be equivalent to a certain “factorization lifting” property which includes both Grothendieck fibrations and opfibrations.
As is evident from the fact that such functors have a name, not every functor is exponentiable in Cat. In particular, although $Cat$ is cartesian closed, it is not locally cartesian closed.
It is easy to write down examples of colimits in $Cat$ that are not preserved by pullback (as they would be if pullback had a right adjoint). For instance, let $\mathbf{2}$ denote the walking arrow, i.e. the ordinal $2$ regarded as a category, $1$ the terminal category, and $\mathbf{3} = \mathbf{2} \sqcup_1 \mathbf{2}$ the ordinal $3 = (a \to b \to c)$ regarded as a category. Then the pushout square
in the slice category $Cat/\mathbf{3}$ pulls back along the inclusion $\mathbf{2}\to \mathbf{3}$ of the arrow $(a\to c)$ to the square
which is certainly not a pushout.
One way to describe the problem is that the pushout has “created new morphisms” that didn’t exist before. But another way to describe the problem is that the inclusion $\mathbf{2}\to\mathbf{3}$ fails to notice that the morphism $(a\to c)$ acquires a new factorization in $\mathbf{3}$ which it didn’t have in $\mathbf{2}$. Conduché‘s observation was that this latter failure is really the only problem that can prevent a functor from being exponentiable.
A functor $p\colon E\to B$ is a strict Conduché functor if for any morphism $\alpha\colon a\to b$ in $E$ and any factorization $p a \overset{\beta}{\to} c \overset{\gamma}{\to} p b$ of $p \alpha$ in $B$, we have:
there exists a factorization $a \overset{\tilde{\beta}}{\to} d \overset{\tilde{\gamma}}{\to} b$ of $\alpha$ in $E$ such that $p \tilde{\beta} = \beta$ and $p \tilde{\gamma} = \gamma$, and
any two such factorizations in $E$ are connected by a zigzag of commuting morphisms which map to $id_c$ in $B$.
(Here, ‘commuting morphism’ means a morphism $d \to d'$ in $E$ such that the pair of triangles in
commute.)
The theorem is then that the following are equivalent:
By “exponentiable in the strict 2-category $Cat$” we mean that pullback along $p$ has a strict right 2-adjoint (i.e. a $Cat$-enriched right adjoint). Of course, this implies ordinary exponentiability in the 1-category $Cat$, while the converse follows via an argument involving cotensors with $\mathbf{2}$ in $Cat$.
For exponentiability in the weak 2-category $Cat$, in the sense of pullback having a weak/pseudo 2-adjoint, we can simply weaken the condition. We say that $p\colon E\to B$ is a (weak) Conduché functor if for any morphism $\alpha\colon a\to b$ in $E$ and any factorization $p a \overset{\beta}{\to} c \overset{\gamma}{\to} p b$ of $p \alpha$ in $B$, we have:
there exists a factorization $a \overset{\tilde{\beta}}{\to} d \overset{\tilde{\gamma}}{\to} b$ of $\alpha$ in $E$, and an isomorphism $p d \cong c$, such that modulo this isomorphism $p \tilde{\beta} = \beta$ and $p \tilde{\gamma} = \gamma$, and
any two such factorizations in $E$ are connected by a zigzag of commuting morphisms which map to isomorphisms in $B$.
A functor can then be shown to be a weak Conduché functor if and only if it is exponentiable in the weak sense in $Cat$.
The Conduché criterion can be reformulated in a more conceptual way by analogy with Grothendieck fibrations. We first observe that to give a functor $p\colon E\to B$ is essentially the same as to give a normal lax 2-functor $B\to Prof$ from $B$ to Prof, the 2-category of profunctors. The latter is also known as a displayed category; see there for more on this correspondence.
Specifically, given a functor $p$, we define $B\to Prof$ as follows. Each object $b\in B$ is sent to the fiber category $p^{-1}(b)$ of objects lying over $b$ and morphism lying over $1_b$. And each morphism $f\colon a\to b$ in $B$ to the profunctor $H_f\colon p^{-1}(a) ⇸ p^{-1}(b)$ for which $H_f(x,y)$ is the set of arrows $x\to y$ in $E$ lying over $f$. The lax structure maps $H_f \otimes H_g \to H_{g f}$ are given by composition in $E$. The converse construction of a functor $p$ from a normal lax 2-functor into $Prof$ is an evident generalization of the Grothendieck construction. Now we can say that:
Thus Conduché functors into $B$ correspond to pseudofunctors from $B$, regarded as a locally discrete bicategory, to the bicategory $Prof$. However, morphisms between Conduché functors over $B$ do not correspond to pseudonatural transformations between such pseudofunctors. To get the correct transformations, we must instead regard $B$ as a vertically discrete double category, and $Prof$ as a pseudo double category with profunctors horizontally and functors vertically; then pseudo double functors $B\to Prof$ again correspond to Conduché functors into $B$, and vertical double transformations between them correspond to functors between Conduché functors into $B$.
More generally, the slice category $Cat/B$ is equivalent to the hom-category $Dbl_{normal,lax}(B,Prof)$, with its full subcategory consisting of Conduché functors corresponding to the pseudo double functors.
Non-strict Conduché functors and Street fibrations may be equivalently characterized by an “up-to-iso” version of the above constructions using essential fibers.
Ayala and Francis prove an analogous characterization of exponentiable (∞,1)-functors. The (∞,1)-categorical context eliminates the “level-shifting” in the characterization via $Prof$ (i.e. the presence of a bicategory Prof when discussing only exponentiable 1-functors). Thus, there is an (∞,1)-category (∞,1)Prof such that exponentiable $(\infty,1)$-functors into an $(\infty,1)$-category $B$ correspond to $(\infty,1)$-functors $B\to (\infty,1)Prof$.
As in the 1-categorical case, ordinary $(\infty,1)$-transformations between functors $B\to (\infty,1)Prof$ do not give the correct maps between exponentiable $(\infty,1)$-functors over $B$; we need to instead regard $(\infty,1)Prof$ as a sort of “$(\infty,1)$-double category”. Ayala and Francis consider only the vertically-invertible fragment of this $(\infty,1)$-double category, which can be represented as a functor from an $\infty$-groupoid to an $(\infty,1)$-category (a sort of proarrow equipment with all 2-cells and all 1-cells in the domain invertible); this is what they call a “flagged” $(\infty,1)$-category and is also what is represented by a non-complete Segal space. Of course, restricting to the vertically-invertible fragment of $(\infty,1)Prof$ also restricts what it classifies to the $\infty$-groupoid of exponentiable $(\infty,1)$-functors over $B$ rather than the whole $(\infty,1)$-category thereof.
The above considerations show that any Grothendieck fibration or opfibration is a (strict) Conduché functor, while any Street fibration or opfibration is a non-strict Conduché functor.
If $\mathbf{2}$ denotes the interval category, then any normal lax functor out of $\mathbf{2}$ is necessarily pseudo, since there are no composable pairs of nonidentity arrows in $\mathbf{2}$. It follows that, as pointed out by Jean Benabou, any functor with codomain $\mathbf{2}$ is a Conduché functor. Note that functors with codomain $\mathbf{2}$ can also be identified with profunctors, the two fiber categories being the source and target of the corresponding profunctor.
As with exponentiable morphisms in any category, Conduché functors are closed under composition.
J. Giraud, Méthode de la descente, Bull. Math. Soc. Mémoire 2 (1964). (numdam)
F. Conduché, Au sujet de l’existence d’adjoints à droite aux foncteurs ‘image reciproque’ dans la catégorie des catégories , C. R. Acad. Sci. Paris 275 Série A (1972) pp.891-894. (gallica)
The definitions and proofs of the above theorems, along with the 2-categorical generalization (Conduché considered only the 1-categorical case) can also be found in
A description of the characterization in terms of lax normal functors can be found in
Discrete Conduché functors are considered in
M. Bunge, S. Niefield, Exponentiability and single universes , JPAA 148 (2000) pp.217-250.
Peter Johnstone, A Note on Discrete Conduché Fibrations , TAC 5 no.1 (1999) pp.1-11. (pdf)
An analogue of Conduché functors for ∞-categories, classified by maps into an ∞-category version of Prof, is studied in
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