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Torsion on elliptic curves in isogeny classes
Authors:Yasutsugu Fujita   Tetsuo Nakamura
Affiliation:Mathematical Institute, Tohoku University, Sendai 980-8578, Japan ; Mathematical Institute, Tohoku University, Sendai 980-8578, Japan
Abstract:
Let $ E$ be an elliptic curve over a number field $ K$ and $ mathcal C$ its $ K$-isogeny class. We are interested in determining the orders and the types of torsion groups $ E(K)_{textrm{tors}}$ in $ mathcal C$. For a prime $ l$, we give the range of possible types of $ l$-primary parts $ E(K)_{(l)}$ of $ E(K)_{textrm{tors}}$ when $ E$ runs over $ mathcal C$. One of our results immediately gives a simple proof of a theorem of Katz on the order $ sup_{E in mathcal C}vert E(K)_{(l)}vert$ of maximal $ l$-primary torsion in $ mathcal C$.

Keywords:Elliptic curve   torsion   isogeny
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