1.2. SIMPLICITY, ISOTYPICITY, AND ENDOMORPHISM ALGEBRAS 15
the Main Theorem of Complex Multiplication) are discussed in Chapter 2, and
Appendix A provides proofs of the Main Theorem of Complex Multiplication and
some results of Tate over finite fields.
1.2. Simplicity, isotypicity, and endomorphism algebras
1.2.1. Simple abelian varieties. An abelian variety A over a field K is simple
(over K) if it is non-zero and contains no non-zero proper abelian subvarieties.
Simplicity is not generally preserved under extension of the base field; see Example
1.6.3 for some two-dimensional examples over finite fields and over Q. An abelian
variety A over K is absolutely simple (over K) if AK is simple.
1.2.1.1. Lemma. If A is absolutely simple over a field K then for any field ex-
tension K /K, the abelian variety AK over K is simple.
Proof. This is an application of direct limit and specialization arguments, as we
now explain. Assume for some K /K that there is a non-zero proper abelian sub-
variety B AK . By replacing K with an algebraic closure we may arrange that
K and then especially K is algebraically closed. (The algebraically closed property
for K is unimportant, but it is crucial that we have it for K.) By expressing K
as a direct limit of finitely generated K-subalgebras, there is a finitely generated
K-subalgebra R K such that B = BK for an abelian scheme B Spec(R)
that is a closed R-subgroup of AR.
The constant positive dimension of the fibers of B Spec(R) is strictly less
than dim(A), as we may check using the K -fiber B AK . Since K is algebraically
closed we can choose a K-point x of Spec(R). The fiber Bx is a non-zero proper
abelian subvariety of A, contrary to the simplicity of A over K.
For a pair of abelian varieties A and B over a field K,
Hom0(AK
, BK ) can be
strictly larger than
Hom0(A,
B) for some separable algebraic extension K /K. For
example, if E is an elliptic curve over Q then considerations with the tangent line
over Q force
End0(E)
= Q, but it can happen that
End0(EL)
= L for an imaginary
quadratic field L (e.g., E :
y2
=
x3
x and L = Q(

−1)).
Scalar extension from number fields to C or from an imperfect field to its perfect
closure are useful techniques in the study of abelian varieties, so there is natural
interest in considering ground field extensions that are not separable algebraic (e.g.,
non-algebraic or purely inseparable). It is an important fact that allowing such
general extensions of the base field does not lead to more homomorphisms:
1.2.1.2. Lemma (Chow). Let K /K be an extension of fields that is primary (i.e.,
K is separably algebraically closed in K ). For abelian varieties A and B over K,
the natural map Hom(A, B) Hom(AK , BK ) is bijective.
Proof. See [23, Thm. 3.19] for a proof using faithfully flat descent (which is
reviewed at the beginning of [23, §3]). An alternative proof is to show that the
locally finite type Hom-scheme Hom(A, B) over K is ´etale.
We shall be interested in certain commutative rings acting faithfully on abelian
varieties, so we need non-trivial information about the structure of endomorphism
algebras of abelian varieties. The study of such rings rests on the following funda-
mental result.
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