EN
We study relationships between sequence entropy and the Kronecker and rigid algebras. Let (Y,𝓨,ν,T) be a factor of a measure-theoretical dynamical system (X,𝓧,μ,T) and S be a sequence of positive integers with positive upper density. We prove there exists a subsequence A ⊆ S such that $h^{A}_{μ}(T,ξ|𝓨) = H_{μ}(ξ|𝓚(X|Y))$ for all finite partitions ξ, where 𝓚(X|Y) is the Kronecker algebra over 𝓨. A similar result holds for rigid algebras over 𝓨. As an application, we characterize compact, rigid and mixing extensions via relative sequence entropy.