We determine all measurable functionsI,G,L:[0,1]→ℝsatisfying the functional equation∑i=1n∑j=1mI(pi,qj)=∑i=1n∑j=1mG(pi)I(qj)+∑i=1n∑j=1mL(qj)I(pi)forP∈Γn,Q∈Γmand for a fixed pair(n,m),n≥3,m≥3, whereG(0)=L(0)=0andG(1)=L(1)=1. This functional equation has interesting applications in information theory.