Basically, I'm aware of "splitting principles" for the following three objects (which are all isomorphic modulo torsion).
1. The Chow group a la Fulton.
2. The classical Grothendieck group of vector bundles or coherent sheaves.
3. The $\gamma$-graded Grothendieck group.
I was just wondering where the idea of "the splitting principle" comes from. I'm guessing somewhere in topology when one wanted to define Chern classes and show some properties. But I don't know.
And above that, is there some more general way of looking at this? I know there is a theorem that connects higher K-groups with Chow groups in a sense. So I ask, is there a way of deducing the splitting principle for one of the above objects from the other? (It's easy if we want to do this modulo torsion, of course.)
Best Answer
We can think of the splitting principle as a condition on a "cohomology theory" (of some sort) $E^*$, coming about when working with Chern classes for instance, and then ask: When does $E^*$ satisfy this condition? First, let's make the condition more precise and reformulate it:
Condition 1: Given $X$ and a vector bundle $V$ on $X$, there exists $f: X' \to X$ such that $f^* V'$ has a filtration with subquotients line bundles, and $f^*: E^*(X) \to E^*(X')$ is injective.
But there is a universal choice for $X'$, namely the flag variety of $V$: $p: Fl(V) \to X$. Any $f: X' \to X$ with $f^* V'$ filtered with line bundle subquotients will factor through $p$, and so we're really just asking if $p^*: E^*(X) \to E^*(Fl(V))$ is injective.
Condition 1': For all $X$ and $V$, $p^*: E^*(X) \to E^*(Fl(V))$ is injective.
At this point there are two ways this answer can go, depending on ones tastes:
If your question is one of proof + generalization (which I think it is), rather than vague motivation, then I haven't addressed it yet:
In topology. one can show that any complex-oriented cohomology theory (i.e., one with Chern classes for line bundles) $E^*$ has a projective bundle formula, satisfies all the conditions, etc.
In more-algebro-geometric contexts, you could deduce the Chow + K-theory (I don't know anything about the $\gamma$-filtration) statements by either
One could also ask to generalize this in another direction, replacing vector bundles and $Fl(V)$ by more general $G$-bundles and their associated $G/B$-bundles. In general, that's a more complicated story...