Białynicki-Birula and Motivic Decompositions
Thursday, April 04th, 2013 | Author: Konrad Voelkel
This is about Białynicki-Birula's paper from '72 on actions of reductive linear algebraic groups on non-singular varieties, in particular Gm-operations on smooth projective varieties. I give a proof sketch of Theorem 4.1 therein and explain a little bit how Brosnan applied these results in 2005 to get decompositions of the Chow motive of smooth projective varieties with Gm-operation. Wendt used these methods in 2010 to lift such a decomposition on the homotopy-level, to prove that smooth projective spherical varieties admit stable motivic cell decompositions. Most of this blogpost consists of an outline of the B-B paper.
Białynicki-Birula's algebraic Morse theory
The paper is essentially about algebraic torus actions on varieties and relating the induced action on the tangent space of a fixed point to the variety itself. The most simple torus is just the multiplicative group (think of
or
). In classical Morse theory, one considers "Morse functions", which are a particular kind of function
, and their gradient flow, which is the flow associated to the gradient vector field. Such a flow is nothing but a
-action! Where the Morse-theory people look at smooth manifolds and apply the exponential function from the tangent space (of a critical point of the Morse function, i.e. a fixed point of the flow) to the whole space
, an algebraic geometer has to do something else (as the exponential function is not algebraic). This something else is a gimmick invented by Białynicki-Birula. With this gimmick, a
-action with isolated fixed points provides a cell decomposition, like the CW decomposition from classical Morse theory.
Proof outline
We work over an algebraically closed field . Let
be a quasi-affine algebraic scheme and
a nonsingular closed point. We denote by
a reductive algebraic group, though in the end only the 1-dimensional torus
is relevant.
Given a -action on a scheme
with fixed point
, the tangent space
gets a natural
-action. For any vector space
with
-action there is a decomposition
into the weight-graded pieces. I call the action definite if either the minus- or the plus-part vanishes, and fully definite if also the zero-part vanishes.
Theorem 2.1: Given a reductive group acting on
with a closed irreducible
-invariant subscheme
containing a closed fixed point
nonsingular in
and
, to any
-invariant subspace
of the tangent space
that contains
one can find a closed irreducible
-invariant subscheme
that contains
and has the prescribed tangent space.
(This is what I consider a replacement for the exponential function).
Proof idea: The maximal ideal corresponding to
maps
-equivariant surjective to
. Denote by
the tangent space of
and by
the ideal corresponding to
. Since
is reductive, there exists a
-submodule
that maps isomorphically to
. Then
is an ideal in
, so the corresponding closed subscheme of
has an irreducible component
containing
. By construction,
.
Uniqueness of the subspace is also discussed in Theorem 2.2, in particular we have a Corollary to Theorem 2.2: Let act on
with fixed point
. If
is either the positive, the negative, the non-negative, the non-positive or the zero-part of the graded vector space
, then there exists exactly one closed, irreducible and reduced
-invariant subscheme
through
such that
is non-singular and
.
There is a morphism-version of Theorem 2.1, which is slightly weaker. Roughly, to a -isomorphism of some tangent spaces of two
-schemes, you get a third scheme with étale maps to the two others, and if you already have a
-isomorphism on subschemes, this is taken into account. The precise statement is
Theorem 2.4: Given for sequences
of closed immersions of
-invariant subschemes of quasi-affine algebraic
-schemes and a
-isomorphism
, such that the
-modules
and
are isomorphic, there exists such a sequence
and étale
-morphisms
that map
onto an open subscheme of
.
Proof idea: Inside embed
as
and apply Theorem 2.1 to get a subscheme
that contains
and
, with
. The projections to the factors
are étale at
, hence over a smaller subscheme
that still contains
. Denote by
the union of the preimages of the
in
, then
and
do the job.
The local structure of affine "cells" comes from
Theorem 2.5: For any torus acting on
such that
is a fixed point and the induced action on
is definite, there exists a
-invariant open neighborhood
of
which is
-isomorphic to
, with
a finite-dimensional fully definite
-module.
Proof idea: The arises as the complement of
.
WLOG (as one can show) is reduced, irreducible and
acts effectively. Apply Theorem 2.4 to
,
,
,
,
,
, then the resulting
are not only étale, but also birational (as one can show), hence open immersions. Then
contains an open subscheme
which is
-isomorphic to
for
some open beighbourhood of
in
and
gives the statement of the theorem.
In the full proof of Theorem 2.5, the notion of a universal domain is frequently used. This is a device to handle generic points without talking about prime ideals, which I explained in this blog posts about points.
Given a -rep
one defines the notion of an
-fibration
, which carries a
-action on
and Zariski-locally on
looks like
, with
-action induced by
. We call
the dimension of the
-fibration.
One should remark that an -fibration needn't be a vector bundle, since there might be more
-equivariant automorphisms of
than the linear ones.
The following gives us a uniqueness property for -fibration-structures on maps
.
Corollary to Proposition 3.1: For any torus acting on a nonsingular
, two
-representations
for
and
-fibrations
(respectively), then
is equivalent to
. If furthermore
is a
-fixed point and
is definite, then
.
Proof idea: For any closed point , as
-modules,
, so the
are equivalent. Note that the
in
is uniquely determined, since there is no nonzero
-homomorphism
. By (the corollary to) Theorem 2.2 there exists exactly one
-invariant subscheme
with
nonsingular and
, but
and
both fulfill these conditions, hence
. This shows
.
We call a morphism with
-action on
a
-fibration if it is Zariski-locally over
an
-fibration
for some
-reps
. If the dimensions of the
all coincide, we call that number the dimension of the
-fibration.
Now, let and
any non-singular reduced algebraic
-scheme that can be covered by
-invariant quasi-affine open subschemes (for example any smooth projective
will do, maybe normal quasiprojective suffices, by Sumihiro's equivariant compactification).
Theorem 4.1: Let be the decomposition into connected components. For any
there exists a unique locally closed
-invariant subscheme
and a unique morphism
such that
is a retraction, i.e.
is a closed subscheme of
and
is the identity,
is a
-fibration,
- for any closed fixed point
, the tangent space is
and the dimension of the
-fibration
is
.
Proof idea: Let . By Theorem 2.1 there exists a closed
-invariant irreducible subscheme
with
nonsingular and
. By Theorem 2.5, there is an open
-stable nonsingular subscheme
that still contains
and
is a trivial
-fibration. Using the Corollary to Theorem 2.2 and the Corollary to Proposition 3.1 (the uniqueness statements) we know for
that
and for any third fixed point
we have
.
Since every is noetherian, we find
such that
, so
is a
-invariant, locally closed subscheme of
and a
-fibration
can be uniquely glued together from the
.
Actually, there is also a minus-decomposition, where you use instead. The interplay of these two decompositions for the same
-action is explained in Theorem 4.2: Let
act on a quasi-affine
. For a rational point
, the orbit closure
intersects a connected component
in a non-empty set iff
or
. Moreover,
for all connected components.
Proof idea: The direction is clear. For the other direction apply Theorem 2.4 to
,
,
,
,
,
. For
we have
. Only one
contains
and one can show (using again Theorem 2.1 and 2.2) that actually
and
.
Moreover, from we get
.
Theorem 4.3: Let act on a complete nonsingular algebraic
-scheme
, with
the decomposition of the fixed points into connected components. Then there exists a unique locally closed
-invariant decomposition
and
-fibrations
such that
and for any closed fixed point
,
.
Proof idea: Take the plus-decomposition from Theorem 4.1, then what's missing for the statement ( for
and
) follows from analyzing orbit closures (that is actually Theorem 4.2 together with Lemma 4.1 which I didn't include in this summary).
From this follows Theorem 4.4: If the -action in Theorem 4.3 has isolated fixed points, then any
is isomorphic to an affine space
.
(This looks like a cell structure!)
The proofs and results have been improved a little bit (Hesselink removed the assumption that is algebraically closed), so that the current level of generality provides the following
Theorem:
Let be a smooth projective
-variety over a field
. Then
1) is a smooth closed subscheme of
(Iversen),
2) Given the connected components , there is a filtration
and affine fibrations
,
3) The relative dimension of is the dimension of
for any
.
I want to remark that any generalization of this theorem to quasiprojective or singular situations would be a very impressive result.
The only generalizations I know of are papers of Skowera and Choudhury on Deligne-Mumford stacks and papers of Carrell and Sommese on the Kähler analogue.
Karpenko's, Chernousov-Gille-Merkurjev's and Brosnan's motivic decompositions
Theorem (Karpenko):
Let be a smooth projective variety over a field
, equipped with a filtration
where the
are closed subvarieties, and affine fibrations
of relative dimension
. Then the Chow motive decomposes
.
Corollary (Brosnan):
Let be a smooth projective
-variety over a field
. Then
, where the
are the connected components of the fixed point locus
.
From this, Brosnan re-proved
Theorem (Chernousov-Gille-Merkurjev):
For a projective homogeneous variety (for a reductive group) over a field
, the kernel of the map
consists only of nilpotent elements.
Brosnan proved more, in particular how the motive of decomposes, using this method:
, where
is length and
is the set of minimal length coset representatives of
, with
the set of roots corresponding to
and
the set of roots that are killed by a non-central cocharacter of the maximal torus (taking care of the possible non-splitness of the maximal torus).
Wendt's cellular decomposition of the stable motivic homotopy type
Using the BB-decomposition, one gets a decomposition of the motive. Actually, one gets a bit more, namely a decomposition in the stable A¹-homotopy category. This is even more analogous to CW decompositions coming from Morse theory.
What you need for a cellular decomposition (in the homotopy-theoretic sense), but what's missing in a direct sum decomposition of the motive, are the gluing maps. One has to extract these gluing maps from the BB-decomposition. This was done by Wendt, who used this approach to show stable cellularity of connected split reductive groups and their classifying spaces, as well as stable cellularity of smooth projective spherical varieties under connected split reductive groups.
As this post is already too long, I might explain the motivic cell structures in another post. Actually, you can just take a look at the preprint.
It remains to see how these cell structures look like explicitly!