88 1. ALGEBRAIC THEORY OF COMPLEX MULTIPLICATION
a CM field L ⊂
:= Q ⊗Z End(A) satisfying [L : Q] = 2g, and an
isomorphism φ : AFq B as abelian varieties over Fq.
• (R) CM lifting after finite residue field extension: there is a local domain R
with characteristic 0 and residue field κ of finite degree over Fq, an abelian
scheme A over R with relative dimension g equipped with an action (in the
isogeny category over R) by a CM field L with [L : Q] = 2g, and an isomor-
phism φ : Aκ B ×Spec(Fq) Spec(κ) as abelian varieties over κ.
• (I) CM lifting up to isogeny: there is a local domain R with characteristic 0
and residue field Fq, an abelian scheme A over R with relative dimension g
equipped with an action (in the isogeny category over R) by a CM field L
with [L : Q] = 2g, and an isogeny AFq → B of abelian varieties over Fq.
• (IN) CM lifting to normal domains up to isogeny: there is a normal local
domain R with characteristic 0 and residue field Fq such that (I) is satisfied
for B using R.
• (RIN) CM lifting to normal domains up to isogeny after finite residue field
extension: there is a normal local domain R with characteristic 0 and residue
field κ of finite degree over Fq such that (R) is satisfied for B using R except
that φ is only required to be an isogeny rather than an isomorphism.
• (sCML) strong CM lifting: For every CM field L ⊂
with [L : Q] = 2g
such that OL ⊂ End(B), the abelian variety B satisfies (CML) using a lifting
A such that the Q-subalgebra
220.127.116.11. Remark. By expressing a local ring as a direct limit of local subrings
essentially of finite type over Z, in the formulation of (R) there is no loss of generality
in replacing κ with an algebraic closure of Fq or allowing κ to vary over all extensions
of Fq. Likewise, the normality condition in (RIN) is irrelevant because it can be
attained at the cost of a finite residue field extension (by a specialization argument
that we will give in 2.1.1), and in (IN) we can assume R is complete since essentially
finite type Z-algebras are excellent (ensuring that normality is preserved under
completion of such rings along an ideal). Even in (I) we can assume R is complete
local noetherian since we may first descend to a local noetherian domain R0 ⊂ R
of characteristic 0 with residue field Fq, and then note that the completion R0 of
R0 has a minimal prime of residue characteristic 0 (as R0 → R0 is faithfully flat).
By Remark 18.104.22.168, (RIN) has an aﬃrmative answer for any isotypic B over
Fq, and the CM lift can be chosen using any CM maximal commutative subfield
There are several refinements we wish to answer:
(1) Is a residue field extension necessary? That is, does (IN) hold for every B?
(2) If (IN) does not hold for every B, can we characterize when it holds? And
how about (I) in general (i.e., drop normality, but permit an isogeny without
increasing the residue field)?
(3) Is an isogeny necessary? That is, does (R) hold for every B (requiring the
local domain R to be normal is not a constraint, since we are allowing a finite
extension on κ; cf. Remark 22.214.171.124), or does even (CML) hold for every B?
These questions can be made more specific in several respects. For example,
since the Q-simple
is usually non-commutative, it generally contains more
than one CM maximal commutative subfield L (up to conjugacy) and so we can
pose the CM lifting questions requiring an order in a particular choice of L to lift