Functional equations on abelian groups with involution

Summary We produce complete solution formulas of selected functional equations of the formf(x +y) ±f(x + σ (ν)) = Σ _I ² =₁ g _l (x)h _l (y),x, y∈G, where the functionsf,g ₁,h ₁ to be determined are complex valued functions on an abelian groupG and where σ:G→G is an involution ofG. The special case of σ=−I encompasses classical functional equations like d’Alembert’s, Wilson’s first generalization of it, Jensen’s equation and the quadratic equation. We solve these equations, the equation for symmetric second differences in product form and similar functional equations for a general involution σ.     

On multiplicative maps

No abstract. November 15, 1999     

A link between Wilson’s and d’Alembert’s functional equations

If \({f, g : G \to \mathbb{C}}\), f ≠ 0, is a solution of Wilson’s functional equation on a group G, then g is a d’Alembert function.     

Extensions of the sine addition law on groups

Aequationes mathematicae - Let G be a group, and let $$\chi $$ and $$\mu $$ be characters of G. We find the solutions of the functional equation $$f(xy) = f(x)\chi (y) + \mu (x)f(y)$$ , $$x,y \in...     

The cosine addition law with an additional term

We find the solutions of the cosine addition law with an additional term, including the case of the cosine addition law on semigroups. They are expressed in terms of multiplicative functions and solutions of the sine addition law, apart from an obvious exception. On groups multiplicative and additive functions suffice. As an application we solve extensions of Butler’s problem on semigroups.