Double cosets in free groups

In this paper we study double cosets of finite rank free groups. We focus our attention on cancellation types in double cosets and their formal language properties.Comment: Minor change

Algebraic geometry over algebraic structures II: Foundations

In this paper we introduce elements of algebraic geometry over an arbitrary algebraic structure. We prove Unification Theorems which gather the description of coordinate algebras by several ways.Comment: 55 page