An extension of a variant of d’Alemberts functional equation on compact groups

All paper is related with the non-zero continuous solutions f : G → ℂ of the functional equation f(xσ(y))+f(τ(y)x)=2f(x)f(y),x,y∈G,$${\rm{f}}({\rm{x}}\sigma ({\rm{y}})) + {\rm{f}}(\tau ({\rm{y}}){\rm{x}}) = 2{\rm{f}}({\rm{x}}){\rm{f}}({\rm{y}}),\;\;\;\;\;{\rm{x}},{\rm{y}} \in {\rm{G}},$$ where σ; τ are continuous automorphism or continuous anti-automorphism defined on a compact group G and possibly non-abelian, such that σ² = τ² = id: The solutions are given in terms of unitary characters of G: