  We study the $n$-point functions of scalar multi-trace operators in the
$U(N_c)$ gauge theory with adjacent scalars, such as ${\cal N}=4$ super
Yang-Mills, at tree-level by using finite group methods. We derive a set of
formulae of the general $n$-point functions, valid for general $n$ and to all
orders of $1/N_c$. In one formula, the sum over Feynman graphs becomes a
topological partition function on $\Sigma_{0,n}$ with a discrete gauge group,
which resembles closed string interactions. In another formula, a new skeleton
reduction of Feynman graphs generates connected ribbon graphs, which resembles
open string interaction. We define the moduli space ${\cal M}_{g,n}^{\rm
gauge}$ from the space of skeleton-reduced graphs in the connected $n$-point
function of gauge theory. This moduli space is a proper subset of ${\cal
M}_{g,n}$ stratified by the genus, and its top component gives a simple
triangulation of $\Sigma_{g,n}$.
