  Let $\phi(x,y)$ be a continuous function, smooth away from the diagonal, such
that, for some $\alpha>0$, the associated generalized Radon transforms
\begin{equation} \label{Radon} R_t^{\phi}f(x)=\int_{\phi(x,y)=t} f(y) \psi(y)
d\sigma_{x,t}(y) \end{equation} map $L^2({\mathbb R}^d) \to H^{\alpha}({\mathbb
R}^d)$ for all $t>0$. Let $E$ be a compact subset of ${\mathbb R}^d$ for some
$d \ge 2$, and suppose that the Hausdorff dimension of $E$ is $>d-\alpha$. Then
any tree graph $T$ on $k+1$ ($k \ge 1$) vertices is realizable in $E$, in the
sense that there exist distinct $x^1, x^2, \dots, x^{k+1} \in E$ and $t>0$ such
that the $\phi$-distance $\phi(x^i, x^j)$ is equal to $t$ for all pairs $(i,j)$
corresponding to the edges of the graph $T$.
  We also extend this notion to hyper-graphs where points $x^1, x^2, \dots,
x^{k+1}$ are connected by a hyper-edge if $\phi(x^i,x^j)=t_{ij}$ for a set of
pairs $(i,j)$ selected in accordance with a graph $H$, and $\{t_{ij}\}$ is a
pre-assigned collection of positive distances. A chain of hyper-edges is a
sequence $$\{(x^1_1, \dots, x^{k+1}_1), (x^1_2, \dots, x^{k+1}_2), \dots,
(x^1_n,\dots, x^{k+1}_n)\},$$ where $(x^1_i, \dots, x^{k+1}_i)$ and
$(x^1_{i+1}, \dots, x^{k+1}_{i+1})$, $1 \leq i \leq n$, share one and only one
vertex $x^J_i$, with $J$ pre-assigned. A hyper-graph tree is defined
analogously. The key estimate behind these arguments is a variant of
(\ref{Radon}) where the average over the level set of $\phi(x,\cdot)$ is
replaced by the average over an intersection of such surfaces. In the process,
we develop a general graph theoretic paradigm that reduces a variety of point
configuration questions to operator theoretic estimates.
