In his classical paper [1], Deuring proved the following refined result concerning the endomorphism rings of elliptic curves (and much more).
In this chapter, we will prove part of Deuring's theorem. Namely, we will prove that the endomorphism ring of a supersingular elliptic curve over a finite field is an maximal order in the rational quaternion algebra ramified only at and
, using an approach different from Deuring's original proof. Our argument of the maximality comprises two major parts. On the
-part,
can be identified with the endomorphisms of the
-adic Tate module. On the
-part,
can be identified with the endomorphisms of the formal group
.
We will begin with giving a criterion for endomorphism rings in terms of the heights of the formal groups. Next, we will discuss several interesting properties of quaternion algebras. By utilizing the nontrivial results of Tate and Dieudonné-Lubin, we will be able to achieve our goal mentioned above. Finally, two sections will be devoted to a short discussion of abelian varieties and Tate's isogeny theorem.
As promised in the first chapter, we will first connect the supersingularity with the -torsion part of an elliptic curve. We mainly follow [2] in this section.
(a)(b) Suppose
is separable for all
, then
. Consider the natural map
, then
is injective in this case. That is because if
for some
, then
, hence
for all
, which implies
since nonconstant isogeny has finite kernel. Now
is commutative and the map
is injective, so
must be commutative.
(b)(c) Let
be the
-Frobenius map. Since
and
is purely inseparable, if
is purely inseparable, then so is
.
Because is purely inseparable, it factors through the
-Frobenius map
on
, namely
where
is an isomorphism. Therefore
, which means
.
(c)(a) Under condition (c), we claim that there are only finitely many elliptic curves that are isogenous to
. Suppose
is an isogeny. Since the isogeny
on
is purely inseparable and
, we know that the isogeny
on
is also purely inseparable. So
. There are only finitely many
-invariants for
, hence the claim follows.
Now suppose is not supersingular, then
is either
or an imaginary quadratic extension of
. Since there only finitely many
that is isogenous to
, we can find a prime
such that
remains a prime in each
. Since
by Theorem 2, we can find a sequence of cyclic groups
satisfying
. Using [2, III 4.12], we get a separable isogeny
with kernel
. By the claim above again, there exist some positive integers
such that
. Now the natural map
has degree
and has kernel
. Since
is a prime in
, comparing degrees we obtain
where
is an isomorphism. But the kernel of
is not cyclic, a contradiction.
¡õ
The following theorem connects the inseparable degree of an isogeny with the height of the associated map of formal groups in positive characteristic. Combining it with Theorem 2, we will be able to give a characterization of endomorphism rings in terms of the height of the formal group (Theorem 4).
Suppose is the
-Frobenius map, then
. On the other hand, the associated homomorphism
, so
.
Suppose is a separable map, then
and
, where
is the invariant differential. So as a differential on formal group
, we have
. But
([2, IV 4.3]), it follows that
, therefore
.
¡õ
In particular, when
is defined over a finite field.
For the remaining two parts, by Corollary 2, we know that . Let us show that if
, then
is transcendental over
. Otherwise, suppose
. We may assume
is defined over some finite field
. Thus the Frobenius map
, and comparing degrees we obtain that
. Hence
is purely inseparable, a contradiction to
by Theorem 3.
Conversely, if is transcendental, then
is a generic elliptic curve and every elliptic curve over
can be realized by specializing the value of
. By Theorem 1, every imaginary quadratic field in which
splits can occur as an endomorphism algebra and these endomorphism algebras have intersection
. Thus
.
¡õ
We will do a quick summary of the basic properties of quaternion algebras in this section. See [3] for more proofs.
Let be a global field,
be a place of
, then there are only two isomorphism classes of quaternion algebras over the local field
. One is the split case, i.e., the matrix algebra
. Another is the ramified case, i.e., the unique division algebra
of dimension 4.
The quaternion algebra is ramified if and only if
has no solution
in
. By the Hasse-Minkowski Principle, the latter can be determined locally. Moreover, when
, we can use Hilbert symbol
as a calculation tool to determine whether
has a solution in
. Then the Hilbert reciprocity law
ensures that the number of places where
ramifies must be even.
Two quaternion algebras are isomorphic if and only if they are isomorphic locally everywhere. Hence the places where ramifies uniquely determine
. Moreover, an order
in
is maximal if and only if
is maximal for all finite places.
Now we hope to show that the endomorphism ring of a supersingular elliptic curve over a finite field is actually a maximal order. It suffices to prove the maximality for all primes, that is, that is a maximal order in
for all primes
.
This could be hard. But it will be fairly easy if we quote the following two theorems. The first theorem is a special case of Tate's isogeny theorem about abelian varieties (see the next two sections). The second theorem is due to Dieudonné-Lubin ([4, Page 72]).
If is in addition supersingular, then
Using these two strong and useful results, we are now ready to prove the maximality of .
Since is separably closed, using Theorem 7 we know that
is isomorphic to the maximal order in the local quaternion algebra. The theorem follows.
¡õ
Abelian varieties can be viewed as a higher dimensional generalization of elliptic curves. In this section we introduce some basic notions about abelian varieties and state the Tate conjecture and list the results about it. The main references for this and the next sections are [5] and [6].
An abelian variety over is a complete connected group variety over
, or equivalently, a projective connected group variety over
([5, I 6.4]). As we expected, its group structure is automatically commutative ([5, I 1.4]).
Analogously to elliptic curves, an isogeny between two abelian varieties is an homomorphism of abelian varieties which is surjective and has finite kernel. An abelian variety is called simple if it does not contain any nontrivial abelian variety. Every abelian variety is isogenous to a product of simple abelian varieties ([5, I 10.1]).
The multiplication-by- map is an isogeny of degree
, where
. Moreover, it is an étale map if
, where
([5, I 7.2]). Similarly, for a prime
, the
-adic Tate module
of an abelian variety
over
is the inverse limit of the
-torsion part (over
) of
; it is a
-module along with a
-action. So
is a free
-module of rank
([6, 19]). For any two abelian varieties
and
, we have a natural homomorphism of
-modules
The injectivity of Theorem 5 also holds for abelian varieties using similar argument ([5, I 10.5]).
The next corollary follows easily from Theorem 9.
Taking -invariants on both sides of (1) we get an injective homomorphism
Tate [7] conjectured that in many cases it is also an isomorphism.
This conjecture for finite fields was proved by Tate himself [8], now called Tate's isogeny theorem. Zarhin [9] proved the Tate conjecture for function fields of positive characteristic. The number field case was proved by Faltings [10] as one of the steps in proving the Mordell conjecture. Faltings [11] proved the case for function fields of characteristic zero.
There is also a -analog of the
-adic Tate module
called the Dieudonné module
, constructed from the
-divisible group
. Tate conjectured that
This conjecture was proved for finite fields by Tate himself (see [14]), and for function fields over finite fields by de Jong [15]. Under certain conditions, there is an equivalence of categories between -divisible groups and formal groups [16]. In particular, when
is a supersingular elliptic curve (hence without
-torsion), the
-divisible group can be identified with the formal group
.
In this section, we will discuss some basic properties of abelian varieties. We will sketch the main steps of the proof of Tate's isogeny theorem. Before doing that, we will first prove a finiteness theorem on abelian varieties over a finite field, which turns out to be one of the key steps in the proof.
Let be an abelian variety and
be a line bundle on
, then the theorem of square ([5, I 5.5]) ensures that
is a homomorphism where
is the translation-by-
isogeny. Let
be the isomorphism classes of all line bundles
satisfying
for any
. When
is an elliptic curve and
is a divisor on
, then
if and only if
, so this definition is compatible with the earlier
. One can show
actually maps onto
([6, 8]).
There is a pair consisting of a dual abelian variety and a Poincaré bundle which is the solution to the moduli problem of parametrizing
([6, 13]). A polarization of
is an isogeny
such that
for some ample line bundle
on
. If
has degree one, then
is called a principal polarization.
There are several interesting results about line bundles on abelian varieties.
Over a finite field, there are only finitely many isomorphism classes of elliptic curves since there are only finitely many -invariants. This finiteness result can be extended to abelian varieties ([5, I 11.2]).
Let us sketch the major steps of the proof of Tate's isogeny theorem to end this chapter.
Step 2 Let be the subalgebra of
generated by the automorphisms of
defined by elements of
. To prove the isomorphism, it is equivalent to show that
is the commutant of
in
. Since
is semisimple, it suffices to show that
is the commutant of
in
by the von Neumann bicommutant theorem.
Step 3 Let be a polarization of
. Define a pairing
on
by setting
where
is the pairing of
and
induced by the Weil pairing. Let
be a subspace of
which is maximally isotropic with respect to the pairing
and stable under
-action. Use the Finiteness Theorem 11 to construct an endomorphism
such that
.
Step 4 Let be the subalgebra of
generated by the Frobenius endomorphisms of
over
. Show that for any
which splits completely in
, we have
and use Step 3 to show that
is the commutant of
in
.
Step 5 Use the semisimplicity of the Frobenius map on to show that all
's have the same dimension and complete the proof by Step 1.
¡õ
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