A Street fibration, or weak fibration, is a generalization of a Grothendieck fibration which is non-evil, i.e. covariant under equivalence of categories. In Cat, almost all Street fibrations which arise in practice are actually Grothendieck fibrations, so the extra generality is not very useful; since it is also fairly cumbersome, it is rarely used. However, when working with anafunctors or internally to a bicategory (a weak 2-category), one is essentially forced to work with Street fibrations instead; see below.
Let be a functor. The notion of when a morphism of is cartesian is unchanged from the version for Grothendieck fibrations. However, now we say that is a Street fibration if for any and there exists a cartesian and an isomorphism such that .
Back in the 1960s, J. Gray characterized fibrations as the functors where for every the slice functor? has a right adjoint right inverse . One avoids evilness if one just requires that is full and faithful, i.e. the counit of the adjunction is an isomorphism. Explicating this condition in elementary terms, one arrives at the above definition.
A simpler version of this characterization is the following.
A functor is a cloven Street fibration if and only if the canonical functor has a right adjoint in .
First, recall that the 2-category has objects the functors , as morphisms the triangles
which commute up to specified isomorphism, and as 2-cells the natural transformations which commute with the specified isomorphisms (i.e. is the identity).
Next, recall that the comma category has objects the triples , with a morphism in . Let denote the projection . The canonical morphism in sends to the triple .
Somewhat imprecisely, seeing both categories and as sitting over means that functors between those should be the identity (up to specified isomorphism) on the component, and natural transformations should have the identity as their component (modulo the specified isomorphisms).
Now, to give an adjunction in , it suffices to give, for each object in , an object in and an arrow in that is universal from to . Such an arrow consists of a pair of morphisms in and in such that
commutes, i.e. such that .
Note that since , the morphisms supply a natural transformation , not necessarily invertible. By general doctrinal adjunction arguments, in order for to be an adjunction in , it is necessary and sufficient that be invertible (and we then use it to make into a morphism in ).
Now, the universal property of tells us that for any other morphism in from some to , i.e., for any and any pair making the square
commute, there is a unique map in such that the above square factors in as
In other words, the universal property provides a unique such that and , which exactly asserts that is a cartesian arrow.
Thus, the existence of a right adjoint to means precisely that for each morphism , a choice is given of a cartesian lift of up to isomorphism. In turn, this means that is a cloven fibration.
Most other characterizations of Grothendieck fibrations can also be given in a “non-evil” version to characterize Street fibrations.
Every Grothendieck fibration is a Street fibration, as is every equivalence of categories (the latter are not in general Grothendieck fibrations). Since Street fibrations are closed under composition, any functor equivalent to a Street fibration is also a Street fibration (this is the “covariance” property). As remarked above, however, almost all functors in Cat which one wants to treat as “fibrations” are actually Grothendieck fibrations. Moreover, in every Street fibration is the composite of an equivalence with a Grothendieck fibration: the latter can be taken to be a fibrant replacement of it in the canonical model structure on Cat. (In fact, a Street fibration is a Grothendieck fibration precisely when it is also a isofibration.)
For these reasons, Street fibrations in are little-studied. The fact that they exist, and that any Street fibration is equivalent to a Grothendieck one, can be useful in assuaging philosophical worries about the “evilness” of the notion of Grothendieck fibration, but in practice in one usually wants to use Grothendieck fibrations.
However, when internalizing in other 2-categories, covariance becomes more important. This is most obvious when the 2-category in which one wishes to internalize is only a weak 2-category (a bicategory), in which it does not even make much sense to internalize the notion of Grothendieck fibration. (This is even the case in Cat if we define it using anafunctors, as is often important to do in the absence of the axiom of choice.) However, even when internalizing in a strict 2-category, if the strict 2-category itself is “only defined up to biequivalence,” then one is again compelled to use internal Street fibrations, since they are “covariant under biequivalence” of the containing bicategory, whereas Grothendieck fibrations are not.
For example, this is arguably the case in the 2-category Topos, where we have to make the arbitrary choice of whether to define the morphisms to be left-exact left adjoints , right adjoints having a left-exact left adjoint, or adjoint pairs in which is left exact. The three choices give three strict 2-categories which are all biequivalent, but not strictly 2-equivalent; thus it is more sensible to consider internal Street fibrations in than strict Grothendieck ones.
Since Street fibrations are the “minimal covariant modification” of Grothendieck ones, any property enjoyed by Grothendieck fibrations which is itself covariant under equivalence will also be possessed by Street fibrations.
One thing to note is that in general, when working with Street fibrations, one must always replace fibers with essential fibers. In particular, the lifting property of a Street fibration is insufficient for a morphism in to induce a functor between strict fibers, but such a functor is induced between essential fibers. In this way we can reconstruct a pseudofunctor from any Street fibration, and show that the 2-category of Street fibrations is equivalent to that of such pseudofunctors (though not, as in the case of Grothendieck fibrations, strictly 2-equivalent).
Just as every Grothendieck fibration is a strictly exponentiable functor, i.e. an exponentiable morphism in the strict 2-category Cat, every Street fibration is a non-strictly exponentiable functor, i.e. an exponentiable morphism in the weak 2-category Cat.
Of course, any Grothendieck fibration is a Street fibration, but the interest is in those which are not Grothendieck. Some of the applications are internal to 2-categories other than Cat.
Street fibrations in Topos are discussed in the paper of Johnstone below.
The notion of “admissible object” in categorical Galois theory can be expressed in terms of a functor being (partially) a Street fibration, not generally Grothendieck.
The notion was originally defined in
although there it was given in a very abstract form. A more explicit form like that given above can be found in
Some alternate characterizations, and applications in Topos, are discussed in