  Associated to any acyclic cluster algebra is a corresponding triangulated
category known as the cluster category. It is known that there is a one-to-one
correspondence between cluster variables in the cluster algebra and exceptional
indecomposable objects in the cluster category inducing a correspondence
between clusters and cluster-tilting objects.
  Fix a cluster-tilting object T and a corresponding initial cluster. By the
Laurent phenomenon, every cluster variable can be written as a Laurent
polynomial in the initial cluster. We give conditions on T equivalent to the
fact that the denominator in the reduced form for every cluster variable in the
cluster algebra has exponents given by the dimension vector of the
corresponding module over the endomorphism algebra of T.
