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Gluing curves of genus 1 and 2 along their 2-torsion. (English) Zbl 1472.14036

Let \(A\) be an abelian variety over a field \(k\). It is well-known that there is an isogeny decomposition (Poincaré’s Complete Reducibility Theorem) in terms of simple abelian subvarieties \[A \sim B_1^{n_1} \times \cdots \times B_r^{n_r}\] that are pairwise non-isogenous over \(k\). This decomposition is unique up to reordering the factors.
In the case that \(A\) equals the Jacobian variety of a curve \(Z\), there exist algorithms to calculate the aforementioned decomposition in terms of the Jacobians of curves over small extensions of \(k\) whenever possible. The decomposition of the Jacobian of curves has been extensively studied.
The article under review consider a different approach to this problem. The authors aim to develop algorithms to construct an abelian variety \(A\) given factors \(B_i\) as before, in some special cases.
Let X be a curve of genus 1 and let \(Y\) be a curve of genus 2, defined over a base field \(k\) whose characteristic is different from 2. The main result of the paper provides criteria for the existence of a curve \(Z\) over \(k\) whose Jacobian is, up to a special isogeny, isogenous to the products of the Jacobians of \(X\) and \(Y\). The authors also developed algorithms to construct the curve \(Z\) once equations for \(X\) and \(Y\) are given.

MSC:

14H40 Jacobians, Prym varieties
14H25 Arithmetic ground fields for curves
14H30 Coverings of curves, fundamental group
14H45 Special algebraic curves and curves of low genus
14H50 Plane and space curves
14K20 Analytic theory of abelian varieties; abelian integrals and differentials
14K30 Picard schemes, higher Jacobians

References:

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