Published online by Cambridge University Press: 13 June 2017
We present the form of the solutions $f:S\rightarrow \mathbb{C}$ of the functional equation
$$\begin{eqnarray}\mathop{\sum }_{\unicode[STIX]{x1D706}\in K}f(x+\unicode[STIX]{x1D706}y)=|K|f(x)f(y)\quad \text{for }x,y\in S,\end{eqnarray}$$
$f$ satisfies the condition
$f(\sum _{\unicode[STIX]{x1D706}\in K}\unicode[STIX]{x1D706}x)\neq 0$ for all
$x\in S$ ,
$(S,+)$ is an abelian semigroup and
$K$ is a subgroup of the automorphism group of
$S$ .