John V. Ryff "The functional equation $af\left( {ax} \right) + bf\left( {bx + a} \right) = bf\left( {bx} \right) + af\left( {ax + b} \right)$ extensions and almost periodic solutions," Bulletin of the American Mathematical Society, Bull. Amer. Math. Soc. 82(2), 325-327, (March 1976)