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Open Access Publications from the University of California

## AVERAGE OF THE FIRST INVARIANT FACTOR OF THE REDUCTIONS OF ABELIAN VARIETIES OF CM TYPE

• Author(s): Kim, Sungjin
• Advisor(s): Duke, William
• et al.
Abstract

Let $E$ be an elliptic curve defined over $\Q$ and with complex multiplication by $\mO_K$, the ring of integers in an imaginary quadratic field $K$. Let $p$ be a prime of good reduction for $E$. It is known that $E(\F_p)$ has a structure

E(\F_p)\simeq \Z/d_p\Z \oplus \Z/e_p\Z

with uniquely determined $d_p|e_p$.

We give an asymptotic formula for the average order of $e_p$ over primes $p\leq x$ of good reduction, with improved error term $O(x^2/\log^A x)$ for any positive number $A$, which previously $O(x^2/\log^{1/8} x)$ by ~\cite{Wu}. Further, we obtain an upper bound estimate for the average of $d_p$, and a lower bound estimate conditionally on nonexistence of Siegel-zeros for Hecke L-functions.

Then we extend the methods to abelian varieties of CM type. For a field of definition $k$ of an abelian variety $\Av$ and prime ideal $\ip$ of $k$ which is of a good reduction for $\Av$, the structure of $\Av(\F_{\ip})$ as abelian group is:

\Av(\F_{\ip})\simeq \Z/d_1(\ip)\Z\oplus\cdots\oplus\Z/d_g(\ip)\Z\oplus\Z/e_1(\ip)\Z\oplus\cdots\oplus\Z/e_g(\ip)\Z,

where $d_i(\ip)|d_{i+1}(\ip)$, $d_g(\ip)|e_1(\ip)$, and $e_i(\ip)|e_{i+1}(\ip)$ for $1\leq i We use the class field theory and the main theorem of complex multiplication to obtain the average behaviors of$d_1(\ip)$when averaged over primes$\ip$in$k$with$N\ip

Finally, for elliptic curves $E$ over $\Q$, the asymptotic density $C_{E,j}$ of primes $p\leq x$ with $d_p=j$ which is given by ~\cite{C2}:

C_{E,j}=\sum_{k=1}^{\infty} \frac{\mu(k)}{[\Q(E[jk]):\Q]}.

We prove under an appropriate conditions that these constants are positive.

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