We determine the set of geometric endomorphism algebras of geometrically split abelian surfaces
defined over
.
In particular we find that this set has cardinality 92. The essential part of the
classification consists in determining the set of quadratic imaginary fields
with class group
for which there exists
an abelian surface
defined over
which is geometrically isogenous to the square of an elliptic curve with CM by
. We first
study the interplay between the field of definition of the geometric endomorphisms
of and
the field
.
This reduces the problem to the situation in which
is a
-curve
in the sense of Gross. We can then conclude our analysis by employing Nakamura’s
method to compute the endomorphism algebra of the restriction of scalars of a Gross
-curve.
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