Tom Davison mentioned in his talk The functional equation f(x)f(y)f(z) = f(x) + f(y) + f(z) (1) at the 40th International Symposium on Functional Equations, 2002 in Gronów, Poland, the theorem of elementary geometry that f(ξ) = tan(ξ) solves (1) for the angles x, y, z ε I := ]0, π/2 [of a triangle, and he was able to transform (1) equivalently into g(x)g(y)g(z) = 1 under appropriate and motivated conditions, by applying the field of complex numbers.