The coefficient of the D6R4 interaction in the low energy
expansion of the two-loop four-graviton amplitude in type II superstring theory
is known to be proportional to the integral of the Zhang-Kawazumi (ZK)
invariant over the moduli space of genus-two Riemann surfaces. We demonstrate
that the ZK invariant is an eigenfunction with eigenvalue 5 of the
Laplace-Beltrami operator in the interior of moduli space. Exploiting this
result, we evaluate the integral of the ZK invariant explicitly, finding
agreement with the value of the two-loop D6R4 interaction predicted
on the basis of S-duality and supersymmetry. A review of the current
understanding of the D2pR4 interactions in type II superstring
theory compactified on a torus Td with p≤3 and d≤4 is
included.Comment: 40 pages, various typos and coefficients corrected in version