  P. Novikov in 1938 has proved that if $u_1(x)=u_2(x)$ for $|x|>R$, where
$R>0$ is a large number, $$u_j(x):=\int_{D_j}g_0(x,y)dy, \quad g_0(x,y):=\frac
1 {4\pi |x-y|},$$ and $D_j\subset \mathbb{R}^3$, $j=1,2,$ $D_j\subset B_R$, are
bounded, connected, smooth domains, star-shaped with respect to a common point,
then $D_1=D_2$. Here $B_R:= \{x: |x|\le R\}$.
  Our basic results are:
  a) the removal of the assumption about star-shapeness of $D_j$,
  b) a new approach to the problem,
  c) the construction of counter-examples for a similar problem in which $g_0$
is replaced by $g=\frac {e^{ik|x-y|}}{4\pi |x-y|}$, where $k>0$ is a fixed
constant.
