A genus 2 curve $C$ has an elliptic subcover if there exists a degree $n$maximal covering $\psi: C \to E$ to an elliptic curve $E$. Degree $n$ ellipticsubcovers occur in pairs $(E, E')$. The Jacobian $J_C$ of $C$ is isogenous ofdegree $n^2$ to the product $E \times E'$. We say that $J_C$ is $(n, n)$-split.The locus of $C$, denoted by $\L_n$, is an algebraic subvariety of the modulispace $\M_2$. The space $\L_2$ was studied in Shaska/V\"olklein andGaudry/Schost. The space $\L_3$ was studied in Shaska (2004) were an algebraicdescription was given as sublocus of $\M_2$. In this survey we give a brief description of the spaces $\L_n$ for a general$n$ and then focus on small $n$. We describe some of the computational detailswhich were skipped in Shaska/V\"olklein and Shaska (2004). Further weexplicitly describe the relation between the elliptic subcovers $E$ and $E'$.We have implemented most of these relations in computer programs which checkeasily whether a genus 2 curve has $(2, 2)$ or $(3, 3)$ split Jacobian. In eachcase the elliptic subcovers can be explicitly computed.
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机译:如果存在一个度数$ n $,最大覆盖度为\\ psi:从C到E $到一个椭圆曲线$ E $,则属2曲线$ C $具有一个椭圆子覆盖。度为$ n $的椭圆形子覆盖是成对出现的($(E,E')$。 $ C $的Jacobian $ J_C $与产品$ E \ E'$的度数$ n ^ 2 $等价。我们说$ J_C $是$(n,n)$分割的。$ C $的轨迹由$ \ L_n $表示,是模空间$ \ M_2 $的代数子变量。 $ \ L_2 $空间在Shaska / V \“ olklein andGaudry / Schost中进行了研究。空间$ \ L_3 $在Shaska(2004年)中进行了研究,其中代数描述被指定为$ \ M_2 $的子位置。在此调查中,我们给出简要描述了一般$ n $的空间$ \ L_n $,然后着重于小$ n $。我们描述了Shaska / V \“ olklein and Shaska(2004)中忽略的一些计算细节。我们进一步明确地描述了椭圆子覆盖$ E $和$ E'$之间的关系,我们已经在计算机程序中实现了大多数这样的关系,这些程序可以轻松地检查属2曲线是$(2,2)$还是$(3,3) $拆分Jacobian。在每种情况下,都可以显式计算椭圆的子覆盖。
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