On a functional equation of sum form with three unknown functions (Q1192295)

The author obtains the solution of the functional equation \(f(xy)+f(x(1- y))+f((1-x)y)+f((1-x)(1-y))=h(x)h(y)\) where \(f,h:[0,1]\to\mathbb{C}\) (complex numbers) with \(f\) having derivatives of order three, to solve another equation \(F(xy)+F(x(1-y))+F((1-x)y)+F((1-x)(1-y))=G(x)H(y)\).