In this paper the general solution of the functional equation fan ordered, dense, Abelian group, the topology on G is generated by the open intervals of G, the sets Dx, Dy, Dx+y are defined by Dx := {u ∈ G | ∃v ∈ G : (u, v) ∈ D}, Dy := {v ∈ G | ∃u ∈ G : (u, v) ∈ D}, Dx+y := {z ∈ G | ∃(u, v) ∈ D : z = u + v}, and Y (+) is an Abelian group.The main result of the article is a common generalization of similar results by L. Székelyhidi and J. Rimán. Analogous theorem concerning logarithmic functions is also shown.