# A Grothendieck fibration over a weakly contractible category with weakly contractible fibers is weakly contractible?

Let $B$ be a weakly contractible category (that is, it has a weakly contractible nerve), and let $F:E\to B$ be a Grothendieck fibration. Suppose further that the ordinary fibers of the Grothendieck fibration, $F^{-1}(b)$ for each $b\in \mathrm{Ob}(B)$ are themselves weakly contractible. Does it follow that $E$ is weakly contractible?

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Does Quillen's theorem A (ncatlab.org/nlab/show/geometric+realization+of+categories) imply this? It does if the fibers $F^{-1}(b)$ are the same as the geometric realizations of the comma categories $F / b$. – Evan Jenkins Jul 24 '11 at 5:27
I think that because the map is supposed to be a fibration $F^{-1}b$ should be weakly equivalent to $F/b$ so indeed. – Torsten Ekedahl Jul 24 '11 at 6:57
Thanks, your answers sent me in the right direction. – Harry Gindi Jul 24 '11 at 7:02
To elaborate on Torsten's point (also see the reference in my answer): Any functor admitting an adjoint is a weak (Thomason) equivalence, and it is not hard to show that the property of being a prefibration (every map $a\to F(x)$ lifts to a weakly cartesian map $y\to x$ (weak cartesianness is a universal factorization only over identities) is equivalent to showing that the canonical functor from the fiber $F^{-1}(b)\to b\F$, the coslice category, admits a right adjoint. – Harry Gindi Jul 24 '11 at 8:09

The statement there is that given any functor $u:A\to B$ in $\mathrm{Cat}/C$ between (Grothendieck) prefibrations $A\to C$ and $B\to C$ whose fibers $u\times_C c:A\times_C c \to B\times_C c$ for all $c\in \mathrm{Ob}(C)$ are $W$-equivalences with respect to a fixed fundamental localizer $W$ on $Cat$, the functor $u$ is a colocal $W$-equivalence over $C$ (and therefore a $W$-equivalence).
The case in my question then follows by letting $W$ be the minimal fundamental localizer, by letting $C=B$, and letting the prefibration $B\to C$ be the identity map.