Algebraic and F-Independent sets in 2-firs
  Let $R$ denote a 2-fir. The notions of F-independence and algebraic subsets of R are defined. The decomposition of an algebraic subset into similarity classes gives a simple way of translating the F-independence in terms of dimension of some vector spaces. In particular to each element $a \in R$ is attached a certain algebraic set of atoms and the above decomposition gives a lower bound of the length of the atomic decompositions of $a$ in terms of dimensions of certain vector spaces. A notion of rank is introduced and fully reducible elements are studied in details. 
A. Leroy, A. Ozturk