Canonically positive basis of cluster algebras of type $A_2^{(1)}$.

link: http://arxiv.org/abs/0904.2543
Abstract

We study cluster algebras of type $A_2^{(1)}$ with every choice of
coefficients. We find linear basis of such algebras which are called
canonically positive: positive linear combinations of the elements of such
basis coincide with the cone of positive elements of the cluster algebra.