For an Abelian variety A defined over a field F with ring of integers R, consider the Néron model of A, which is a 'best possible' model of A defined over R. This model may be represented as a scheme over
gives back A. Let A0 denote the open subgroup scheme of the Néron model whose fibres are the connected components. For a residue fieldk, A0k is a group variety over k, hence an extension of an abelian variety by a linear group. If this linear group is an algebraic torus, so that A0k is a semiabelian variety, then A has semistable reduction at the prime corresponding to k. If F is global, then A is semistable if it has good or semistable reduction at all primes.
The semistable reduction theorem of Alexander Grothendieck states that an abelian variety acquires semistable reduction over a finite extension of F.
Semistable elliptic curve
A semistable elliptic curve may described more concretely as an elliptic curve that has bad reduction only of multiplicative type. Suppose E is an elliptic curve defined over the rational number field Q. It is known that there is a finite, non-empty setS of prime numbersp, for which E has bad reductionmodulop. This can be explained by saying that the curve Ep obtained by reduction of E to the prime field with p elements has a singular point. Roughly speaking, the condition of multiplicative reduction amounts to saying that the singular point is a double point, rather than a cusp. Which it is, is something effectively computable, according to Tate's algorithm. Therefore in a given case it is decidable whether or not the reduction is semistable, namely multiplicative reduction at worst.
The semistable reduction theorem for E may also be made explicit: E acquires semistable reduction over the extension of F by the coordinates of the points of order 2 and 3.
