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{\bf G. Dupont}
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{\bf Positivity in Coefficient-Free Rank Two Cluster Algebras}
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Let $b,c$ be positive integers, $x_1,x_2$ be indeterminates
over ${\Bbb Z}$ and $x_m, m \in {\Bbb Z}$ be rational functions defined by
$x_{m-1}x_{m+1}=x_m^b+1$ if $m$ is odd and $x_{m-1}x_{m+1}=x_m^c+1$ if
$m$ is even. In this short note, we prove that for any $m,k \in {\Bbb Z}$,
$x_k$ can be expressed as a substraction-free Laurent polynomial in
${\Bbb Z}[x_m^{\pm 1},x_{m+1}^{\pm 1}]$. This proves Fomin-Zelevinsky's
positivity conjecture for coefficient-free rank two cluster algebras.



\bye