Let
be an abelian variety over a global function field
of characteristic
. We study the
-invariant appearing in the
Iwasawa theory of
over
the unramified
-extension
of . Ulmer
suggests that this invariant is equal to what he calls the dimension of the Tate–Shafarevich
group of
and that it is indeed the dimension of some canonically defined group
scheme. Our first result is to verify his suggestions. He also gives a formula
for the dimension of the Tate–Shafarevich group (which is now the
-invariant)
in terms of other quantities including the Faltings height of
and Frobenius slopes of the numerator of the Hasse–Weil
-function
of
assuming the conjectural Birch–Swinnerton-Dyer formula. Our next result is to prove this
-invariant
formula unconditionally for Jacobians and for semistable abelian varieties. Finally, we show
that the “”
locus of the moduli of isomorphism classes of minimal elliptic surfaces endowed with
a section and with fixed large enough Euler characteristic is a dense open
subset.
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