Cosine and Sine Addition and Subtraction Law with an Automorphism

Abstract

Let S be a semigroup. Our main results are that we describe the complex-valued solutions of the following functional equations g(xσ(y))=g(x)g(y)+f(x)f(y),x,y∈S,f(xσ(y))=f(x)g(y)+f(y)g(x),x,y∈S,\matrix{ {g\left( {x\sigma \left( y \right)} \right) = g\left( x \right)g\left( y \right) + f\left( x \right)f\left( y \right),} & {x,y \in S,} \cr {f\left( {x\sigma \left( y \right)} \right) = f\left( x \right)g\left( y \right) + f\left( y \right)g\left( x \right),} & {x,y \in S,} \cr } and f(xσ(y))=f(x)g(y)-f(y)g(x),x,y∈S,\matrix{ {f\left( {x\sigma \left( y \right)} \right) = f\left( x \right)g\left( y \right) - f\left( y \right)g\left( x \right),} & {x,y \in S,} \cr } where σ : S → S is an automorphism that need not be involutive. As a consequence we show that the first two equations are equivalent to their variants. We also give some applications.

Keywords

• functional equation
• semigroup
• addition law
• automorphism
• 39b52
• 39b32