Is an affine fibration over an affine space necessarily trivial? Let $X$ be an algebraic variety over an alg. closed field with zero char. and let $f:X\to \mathbb{A}^n$ be a smooth surjective morphism, such that all fibers (at closed points) are isomorphic to $\mathbb{A}^m$. Does it follow that $X\cong \mathbb{A}^{n+m}$? 
If not, is it true with some additional assumptions? I know that every vector bundle on $\mathbb{A}^n$ is trivial (this is Serre's problem, right?) and that it is even enough to ask it to be locally trivial in etale toplogy. Is every "affine bundle" on $\mathbb{A}^n$ is trivial? I guess it is. The main question is about a general "fibration".
 A: I feel like I already answered this question, but it might have been a variant with fibers isomorphic to tori.  Let the base $B$ be $\mathbb{A}^2$ with coordinates $s$ and $t$.  Begin with $B\times \mathbb{P}^3$, where homogeneous coordinates on $\mathbb{P}^3$ are $[x,y,z,w]$.  Let $S$ be the Cartier divisor in $B\times \mathbb{P}^3$ with defining equation $yz-(sx+tw)^2=0$.  Let $L$ be the Cartier divisor in $S$ with defining equation $y+z-2(sx+tw)=0$.  Let $U$ be the complement of $L$ in $S$.  Then $U$ is affine, the morphism $U\to B$ is smooth, and the fiber over every point other than $(0,0)$ is isomorphic to $\mathbb{A}^2$.  Of course the fiber over $(0,0)$ is isomorphic to a disjoint union of two copies of $\mathbb{A}^2$.  Thus, define $V\subset U$ to be the open subscheme obtained by removing one of these two copies of $\mathbb{A}^2$, i.e., remove the closed subscheme with defining equations $s=t=z=0$.  Then $V$ is quasi-affine, and the affine hull is $U$; this follows by Hartog's theorem / the Riemann extension theorem / S2 extension.  Therefore $V$ is not isomorphic to an affine space.  However, the projection $V\to B$ has all of the requisite properties.    
Edit.  The older answer I mention above was similar, but a little bit different.  That answer was in response to the following similar question, When is a holomorphic submersion with isomorphic fibers locally trivial?.
