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Some finite terms from ladder diagrams in three and four loop maximal supergravity

We consider the finite part of the leading local interactions in the low energy expansion of the four graviton amplitude from the ladder skeleton diagrams in maximal supergravity on T^2, at three and four loops. At three loops, we express the D^8 R^4 and D^{10} R^4 amplitudes as integrals over the moduli space of an underlying auxiliary geometry. These amplitudes are evaluated exactly for special values of the the moduli of the auxiliary geometry, where the integrand simplifies. We also perform a similar analysis for the D^8 R^4 amplitude at four loops that arise from the ladder skeleton diagrams for a special value of a parameter in the moduli space of the auxiliary geometry. While the dependence of the amplitudes on the volume of the T^2 is very simple, the dependence on the complex structure of the T^2 is quite intricate. In some of the cases, the amplitude consists of terms each of which factorizes into a product of two SL(2,Z) invariant modular forms. While one of the factors is a non--holomorphic Eisenstein series, the other factor splits into a sum of modular forms each of which satisfies a Poisson equation on moduli space with source terms that are bilinear in the Eisenstein series. This leads to several possible perturbative contributions unto genus 5 in type II string theory on S^1. Unlike the one and two loop supergravity analysis, these amplitudes also receive non--perturbative contributions from bound states of three D--(anti)instantons in the IIB theory.