XCI.2 (1999)
On elliptic curves in characteristic 2 with wild additive reduction
by
Andreas Schweizer (Montreal)
Introduction. In [Ge1] Gekeler classified all elliptic curves over F 2r(T ) with one rational place of multiplicative reduction (without loss of generality located at ∞), one further rational place of bad reduction (without loss of generality located at 0) and good reduction elsewhere. So these curves have conductor ∞ · T n where n is a natural number (which actually can be arbitrarily large). In [Ge2] he extended his results to characteristic 3.
Roughly, his strategy can be divided into four steps:
1. Using Drinfeld modular curves, determine the places of supersingular reduction of the elliptic curves with such a conductor.
2. This gives control over the zeros and poles of the j-invariants of these curves.
3. Use Tate’s algorithm to calculate the conductors of the “untwisted”
elliptic curves with the possible j-invariants.
4. Control the effect of twisting on the conductor.
In this paper we extend the results in characteristic 2 by allowing one more place of multiplicative reduction, without loss of generality located at T = 1.
Actually we first prove a quite general and semi-explicit form of step 1, namely: Given a finite field F q (of any characteristic), the places of supersin- gular reduction of an elliptic curve E over F q (T ) with multiplicative reduc- tion at ∞ are contained in a finite set S that depends only on the support of the conductor of E. The set S is given in terms of a Drinfeld modular curve. But it is difficult to make this result really explicit, and even in our simple situation this requires circumstantial arguments and modifications.
The author gratefully acknowledges support from CICMA (Centre In- teruniversitaire en Calcul Math´ematique Alg´ebrique) in the form of a post- doctoral position at Concordia University and McGill University.
1991 Mathematics Subject Classification: 11G05, 11G09.
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