Reducible Abelian varieties and Lax matrices for Euler's problem of two fixed centres

Abel’s quadratures for integrable Hamiltonian systems are defined up to a group law of the corresponding Abelian variety $A$. If $A$ is isogenous to a direct product of Abelian varieties $A\cong A_1\times \cdots \times A_k$, the group law can be used to construct various Lax matrices on the factors $A_1,\dots ,A_k$. As an example, we discuss 2-dimensional reducible Abelian variety $A=E_{+}\times E_{-}$, which is a product of 1-dimensional varieties $E_{\pm }$ obtained by Euler in his study of the two fixed centres problem, and the Lax matrices on the factors $E_{\pm }$.