  We study the non-semisimple terms in the geometric side of the Arthur trace
formula for the split symplectic similitude group or the split symplectic group
of rank 2 over any algebraic number field. In particular, we show that the
coefficients of unipotent orbital integrals are expressed by the Dedekind zeta
function, Hecke L-functions, and the Shintani zeta function for the space of
binary quadratic forms.
