2-natural transformation?
Recall that a functor $E \to B$ is called a fibration if its fibres $E_b$ vary (pseudo-)functorially in $b$. Taking fibre to mean strict fibre results in the notion of Grothendieck fibration, while taking it to mean essential fibre gives the notion of Street fibration.
Similarly, a two-sided fibration $A \leftarrow E \to B$ is a pair of functors whose fibres $E(a,b)$ vary functorially in both $a$ and $b$ (contravariantly in one and covariantly in the other).
Let $K$ be a bicategory with finite 2-limits, and recall that fibrations in $K$ may be defined in any of several ways. Each of these has an analogous version for two-sided fibrations.
Recall that (cloven) fibrations $E \to B$ in $K$ are the (pseudo)algebras for a (pseudo) 2-monad $L$ on $K/B$. For a morphism $p \colon E \to A$ in $K$, $L p$ is given by composing the span $A \overset{p}{\leftarrow} E \to 1$ with the canonical span $\Phi A = A \overset{dom}{\leftarrow} A^{\mathbf{2}} \overset{cod}{\to} A$, so that $L p \colon E/p \to A$ is the canonical projection. This can equivalently be described as the comma object $(1_A/p)$. This 2-monad is lax-idempotent, so that $p\colon E\to B$ is a fibration if and only if the unit $p\to L p$ has a left adjoint with invertible counit.
More generally, the same construction gives a 2-monad $L$ on $Span K(B,A)$, whose algebras we call left fibrations. In Cat, a span $C \overset{p}{\leftarrow} H \overset{q}{\to} D$ is a left fibration if $p$ is a cloven fibration whose chosen cartesian lifts are $q$-vertical. (Since we are working bicategorically, “$q$-vertical” means that they map to isomorphisms under $q$.)
Dually, there is a colax-idempotent 2-monad $R$ on each $Span K(B,A)$ whose algebras are called right fibrations, the special case of $Span Cat(D,1)$ yielding cloven opfibrations.
There is then a composite 2-monad $M$ that takes a span $E$ from $B$ to $A$ to $M E = \Phi A \circ E \circ \Phi B$, and $M$-algebras are called two-sided fibrations. Although $M$ is neither lax- nor colax-idempotent, it is still property-like?.
A two-sided Street fibration from $B$ to $A$ in $Cat$ is given by a span $p \colon E \to A$, $q \colon E \to B$ such that
each $i \colon a \to p x$ in $A$ has a $p$-cartesian lift $\kappa_i \colon i^* x \to x$ in $E$ that is $q$-vertical (that is, $E$ is a left fibration)
each $j \colon q x \to b$ in $B$ has a $q$-opcartesian lift $\kappa^j \colon x \to j_! x$ in $E$ that is $p$-vertical ($E$ is a right fibration)
for every cartesian–opcartesian composite $i^* x \to x \to j_! x$ in $E$, the canonical morphism $j_! i^* x \to i^* j_! x$ is an isomorphism.
By the usual theory of distributive laws, an $M$-algebra $m \colon M E \to E$ gives rise to $L$- and $R$-algebras $m \cdot (\Phi A \circ \eta^R_E)$ and $m \cdot (\eta^L_E \circ \Phi B)$, and conversely an $L$-algebra $\ell$ and an $R$-algebra $r$ underlie an $M$-algebra if and only if there is an isomorphism $r \cdot (\ell \circ \Phi B) \cong \ell \cdot (\Phi A \circ r)$ that makes $r$ a morphism of $L$-algebras.
Now given $\ell$ and $r$, because $L$ is colax-idempotent, there is a unique 2-cell $\bar r \colon r \cdot (\ell \circ \Phi B) \Rightarrow \ell \cdot (\Phi A \circ r)$ that makes $r$ a colax morphism of $L$-algebras. So we want to show that in the case of $Cat$, the components of this natural transformation are the canonical morphisms of (3).
The 2-cell $\bar r$ is given by $\ell \cdot (\Phi A \circ r) \cdot (\epsilon \circ \Phi B)$, where $\epsilon$ is the counit of the adjunction $\eta^L_E \dashv \ell$. Its components are thus given, for each $i \colon a \to p x$ in $A$ and $j \colon p x \to b$ in $B$, by first factoring $\kappa^j \kappa_i$ through the opcartesian $i^* x \to j_! i^* x$ and then factoring the result through the cartesian $i^* j_! x \to j_! x$, to obtain exactly the canonical morphism $j_! i^* x \to i^* j_! x$.
If $A \overset{p}{\leftarrow} E \overset{q}{\to} B$ is a two-sided fibration, then the operation sending $(a,b)$ to the corresponding (essential) fiber of $(p,q)$ defines a pseudofunctor $A^{op}\times B \to Cat$. The third condition in Proposition 1 corresponds to the “interchange” equality $(\alpha,1)(1,\beta) = (1,\beta)(\alpha,1)$ in $A^{op}\times B$. We write $Fib(B,A)$ for the 2-category of two-sided fibrations from $B$ to $A$.
Another definition of internal fibration is that a (cloven) fibration in $K$ is a morphism $p\colon E\to B$ such that $K(X,p)\colon K(X,E)\to K(X,B)$ is a (cloven) fibration in $Cat$, for any $X\in K$, and for any $X\to Y$ the corresponding square is a morphism of fibrations in $Cat$. To adapt this definition to two-sided fibrations, we therefore need only to say what is a two-sided fibration in $Cat$. For this we can use the characterization of Proposition 1.
Let $Fib(A) = Fib_K(A)$ denote the 2-category of fibrations over $A\in K$. It is a well-known fact (apparently due to Benabou) that a morphism in $Fib(A)$ is a fibration in $Fib(A)$ if and only if its underlying morphism in $K$ is a fibration. See fibration in a 2-category. Thus, for any fibration $r\colon C\to A$, we have $Fib_{Fib_K(A)}(r) \simeq Fib_K(C)$.
Of course there is a dual result for opfibrations: for any opfibration $r\colon C\to A$ we have $Opf_{Opf_K(A)}(r) \simeq Opf_K(C)$. When we combine variance of iteration, however, we obtain two-sided fibrations.
A span $A \overset{p}{\leftarrow} E \overset{q}{\to} B$ is a two-sided fibration from $B$ to $A$ if and only if 1. $p\colon E\to A$ is a fibration and 1. $(p,q)\colon E\to A\times B$ is an opfibration in $Fib(A)$.
Recall that the projection $A\times B \to A$ is a fibration (and also an opfibration, although that is irrelevant here), and the cartesian 2-cells are precisely those whose component in $B$ is an isomorphism. Therefore, saying that $(p,q)$ is a morphism in $Fib(A)$, i.e. that it preserves cartesian 2-cells, says precisely that $q$ takes $p$-cartesian 2-cells to isomorphisms.
Now $q$ is an opfibration in $K$ iff $E\to (q/1_B)$ has a left adjoint with invertible counit in $K/B$, and $(p,q)$ is an opfibration in $Fib(A)$ iff $E\to ((p,q)/1_{A\times B})$ has a left adjoint with invertible counit in $Fib(A)/(A\times B)$. Of crucial importance is that here $((p,q)/1_{A\times B})$ denotes the comma object calculated in the 2-category $Fib(A)$, or equivalently in $K/A$ (since monadic forgetful functors create limits), and it is easy to check that this is in fact equivalent to the comma object $(q/1_B)$ calculated in $K$.
Therefore, $(p,q)$ is an opfibration in $Fib(A)$ iff $q$ is an opfibration in $K$ and the left adjoint of $E\to (q/1_B)$ is a morphism in $Fib(A)$. It is then easy to check that this left adjoint is a morphism in $K/A$ iff $p$ inverts $q$-opcartesian arrows, and that it is a morphism of fibrations iff the final condition in Proposition 1 is satisfied.
In particular, we have $Fib(B,A) \simeq Opf_{Fib(A)}(A\times B)$. By duality, $Fib(B,A) \simeq Fib_{Opf(B)}(A\times B)$, and therefore $Fib_{Opf(B)}(A\times B) \simeq Opf_{Fib(A)}(A\times B)$, a commutation result that is not immediately obvious.
This result appears in Bourn–Penon; it was noticed independently and recorded here by Mike Shulman.
A two-sided fibration $A \leftarrow E \to B$ in $K$ is discrete if it is discrete as an object of $K/A \times B$. Since discreteness is a limit construction, it is created by monadic forgetful functors; hence this is equivalent to being discrete as an object of the 2-category $Fib(A,B)$ of two-sided fibrations.
For Grothendieck fibrations in Cat, this means the following.
A two-sided discrete fibration is a span $q \colon E \to A$, $p \colon E \to B$ of categories and functors such that 1. each $b \to p e$ in $B$ has a unique lift in $E$ that has codomain $e$ and is in the fiber over $q e$ 1. each $q e \to a$ in $A$ has a unique lift in $E$ that has domain $e$ and is in the fiber over $p e$ 1. for each $f\colon e \to e'$ in $E$, the codomain of the lift of $q f$ equals the domain of the lift of $p f$ and their composite is $f$.
We write
for the full subcategory on the 2-category $Span K(A,B)$ of spans on the 2-sided discrete fibrations. Since a morphism of spans between discrete fibrations is automatically a morphism of fibrations, this is also the full sub-2-category of the 2-category of two-sided fibrations $Fib(A,B)$. And since they are discrete objects, this 2-category is actually (equivalent to) a 1-category.
Given a profunctor $F : B^{op} \times A \to Set$, its collage is the category $K_F$ over the interval category
With $p^{-1}(0) = B$, $p^{-1}(1) = A$, $K_F(b,a) = F(b,a)$ and $K_F(a,b) = \emptyset$ for all $b \in B$, $a \in A$, where
the composite of $b \stackrel{e}{\to} a$ with $a \stackrel{f}{\to} a'$ is given by $F(b,f)(e)$;
the composite of $b \stackrel{g}{\to} b'$ with $b' \stackrel{e'}{\to} a'$ is given by $F(g, a')(e')$.
There is an equivalence of categories
pseudo-natural in $A, B \in Cat$, between profunctors in Set and discrete fibrations from $A$ to $B$, where $E_F$ is the category whose
objects are sections $\sigma : \Delta[1] \to K_F$ of the collage $p : K_F \to \Delta[1]$
morphisms are natural transformations between such sections;
the two projections $A \leftarrow E_F \to B$ are the two functors induced by restriction along $\{0\} \to \Delta[1] \leftarrow \{1\}$.
First we write out $E_F$ in detail. In the following $b, b', \cdots \in B$ and $a,a', \dots \in A$.
The objects of $E_F$ are morphisms
in $K_F$, hence triples $(b \in B, a \in A, e \in F(b,a))$.
Morphisms are commuting diagrams
in $K_F$. We may identify these with pairs $((b \stackrel{g}{\to}b') \in B,(a \stackrel{f}{\to} a') \in A)$ such that
We check that this construction yields a two-sided fibration. The three conditions are
For
an object of $E_F$ and $a \stackrel{f}{\to} a'$ a morphism in $A$, we have that
is the unique lift to a morphism in $E$ that maps to $Id_b$.
Analogously, for
an object of $E_F$ and $b \stackrel{g}{\to} b'$ a morphism in $B$, we have that
is the unique lift to a morphism in $E$ that maps to $Id_{a'}$.
For
an arbitrary morphism in $E_F$, these two unique lifts of its $A$- and its $B$-projection, respectively, are
and
The codomain and domain do match, since $f e = e' g$ by the existence of the original morphism, and their composite is the original morphism
To see that this construction indeed yields an equivalence of categories, define a functor $(A\leftarrow E \to B) \mapsto (F_E : B^{op} \times A \to Set)$ by setting
$F_E(b,a) := E_{b,a}$;
for a morphism $b \stackrel{g}{\to} b'$ let $F_E(g,a') : F_E(b',a') \to F_E(b,a')$ be the function that sends $b' \stackrel{e'}{\to} a'$ to the domain of the unique lift of $b \stackrel{g}{\to} b'$ with this codomain and mapping to $Id_{a'}$;
for a morphism $a \stackrel{f}{\to} a'$ let $F_E(b,f) : F_E(b,a) \to F_E(b,a')$ be the function that sends $b \stackrel{e}{\to} a$ to the codomain of the unique lift of $a \stackrel{f}{\to} a'$ with this domain and mapping to $Id_{b}$;.
One checks that this yields an equivalence of categories.
The category $E_F$ is equivalently characterized as being the comma category of the diagram $B \to K_F \leftarrow A$.
Note that profunctors can also be characterized by their collages, these being the two-sided codiscrete cofibrations; and the collage corresponding to a two-sided fibration is its cocomma object?.
Grothendieck fibration, Street fibration, discrete fibration, fibration in a 2-category, two-sided fibration,
The notion is originally discussed in
Ross Street. Fibrations and Yoneda’s lemma in a 2-category. In Category Seminar (Proc. Sem., Sydney, 1972/1973), pages 104 133. Lecture Notes in Math., Vol. 420. Springer, Berlin, 1974.
Ross Street, Fibrations in bicategories. Cahiers Topologie Géom. Différentielle, 21(2):111–160, 1980. (Corrections in 28(1):53–56, 1987)
Some further discussion of discrete fibrations can be found in
A useful review of discrete fibrations is in