  Let $f \in L_{loc}^1 (\R^n)\cap \mathcal{S}'$, where $\mathcal{S}'$ is the
Schwartz class of distributions, and $$\int_{\sigma (D)} f(x) dx = 0 \quad
\forall \sigma \in G, \qquad (*)$$ where $D\subset \R^n$ is a bounded domain,
the closure $\bar{D}$ of which is diffeomorphic to a closed ball. Then the
complement of $\bar{D}$ is connected and path connected. Here $G$ denotes the
group of all rigid motions in $\R^n$. This group consists of all translations
and rotations.
  It is conjectured that if $f\neq 0$ and (*) holds, then $D$ is a ball. Other
conjectures, equivalent to the above one, are formulated and discussed. Several
new short proofs are given for the earlier proved results.
