A Generalization of Gajda's Equation on Commutative Topological Groups
  In the present paper we deal with the following generalization of the sine-cosine equation \begin{equation*} \int f_1(x+y-t)+f_2(x-y+t) d\mu(t)=g(x)h(y) \end{equation*} for complex valued functions $f_1$, $f_2$, $g$ and $h$ defined on a commutative topological group $G$, where $\mu$ is a complex measure defined on $G$. 
\.Z. Fechner, L. Sz\'ekelyhidi
math.FA