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 SIGMA 11 (2015), 049, 24 pages       arXiv:1409.8177     
https://doi.org/10.3842/SIGMA.2015.049 
Contribution to the Special Issue on New Directions in Lie Theory 
A Combinatorial Formula for Certain Elements of Upper Cluster Algebras
Kyungyong Lee ab, Li Li c and Matthew R. Mills a
 a) Department of Mathematics, Wayne State University, Detroit, MI 48202, USA
 b) Korea Institute for Advanced Study, Seoul, Republic of Korea 130-722
 c) Department of Mathematics and Statistics, Oakland University, Rochester, MI 48309, USA
 
 
Received September 30, 2014, in final form June 22, 2015; Published online June 26, 2015 
Abstract
 
We develop an elementary formula for certain non-trivial elements  of upper cluster algebras. These elements have positive coefficients. We show that when the cluster algebra is acyclic these elements form a basis. Using this formula, we show that each non-acyclic skew-symmetric cluster algebra of rank 3 is properly contained in its upper cluster algebra.
  
 Key words:
cluster algebra; upper cluster algebra; Dyck path. 
pdf (572 kb)  
tex (118 kb)
 
 
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