  The Fomin-Zelevinsky Laurent phenomenon states that every cluster variable in
a cluster algebra can be expressed as a Laurent polynomial in the variables
lying in an arbitrary initial cluster. We give representation-theoretic
formulas for the denominators of cluster variables in cluster algebras of
affine type. The formulas are in terms of the dimensions of spaces of
homomorphisms in the corresponding cluster category, and hold for any choice of
initial cluster.
