Componentwise Geometry and Monodromy of Generalized Lamé Equations

Abstract

We develop a componentwise geometric and monodromy theory for the generalized Lamé equation on an elliptic curve with singularities at $0$ and $\pm p$. Its log-free locus decomposes canonically into two irreducible components, corresponding to the even and non-even symmetries of the potential. Building on the spectral theory of the even component, we construct the hyperelliptic spectral curve, Baker--Akhiezer functions, and addition map of the non-even component, and prove that \[ \degσ_{n,p}^{(1)}=n(n+1). \] After quotienting by the involution $T\mapsto -T$, we show that the non-even spectral curve is naturally isomorphic to the classical Lamé spectral curve, compatibly with the addition map and the rational function $\kappa$. Consequently, every non-even generalized Lamé equation is monodromy equivalent to a unique classical Lamé equation on the same elliptic curve, via the explicit correspondence \[ \widetilde B=T^2-n(n+1)\wp(p). \] Fixing $\tilde{B}$ yields an isomonodromic family as $p$ varies. The correspondence transfers the spectral geometry, finite-gap structure, and finite-monodromy theories, together with the curvature equations on flat tori, to the non-even component. Combining with the Painlevé VI deformation of the even component, we give a componentwise interpretation of the generalized Lamé degeneration.

76 pages