Let $Q$ be an euclidean quiver. Using friezes in the sense of
Assem-Reutenauer-Smith, we provide an algorithm for computing the (canonical)
cluster character associated to any object in the cluster category of $Q$. In
particular, this algorithm allows to compute all the cluster variables in the
cluster algebra associated to $Q$. It also allows to compute the sum of the
Euler characteristics of the quiver grassmannians of any module $M$ over the
path algebra of $Q$.
Let $\mathcal C$ be a Hom-finite triangulated 2-Calabi-Yau category with
constructible cones and let $T$ be a cluster-tilting object in $\mathcal C$. We
introduce a set $\mathcal G^T(\mathcal C)$ of generic values of the cluster
character associated to $T$ parameterized by the Grothendieck group $K_0(\add
T)$. We prove that the set $\mathcal G^T(\mathcal C)$ naturally contains the
cluster monomials of the cluster algebra associated to the Gabriel quiver of
the cluster-tilted algebra $\End_{\CC}(T)^{\op}$.
We introduce a positive character on the category of finite dimensional
representations of the $\mathbb A$-double-infinite quiver. We prove several
interactions between this character and generalized Chebyshev polynomials
finding applications to acyclic cluster algebras.
If $Q$ is any representation-infinite acyclic quiver, we prove that the
positivity of the cluster character of an indecomposable regular $\kQ$-module
can be deduced from the positivity of the cluster characters of its
quasi-composition factors.
Sherman-Zelevinsky and Cerulli constructed canonically positive bases in
cluster algebras associated to affine quivers having at most three vertices.
These constructions involve cluster monomials and Chebyshev polynomials of the
first kind evaluated at a certain "imaginary" element in the cluster algebra.
We introduce a multivariate generalization of normalized Chebyshev
polynomials of the second kind. We prove that these polynomials arise in the
context of cluster characters associated to Dynkin quivers of type $\mathbb A$
and representation-infinite quivers. This allows to obtain a simple
combinatorial description of cluster algebras of type $\mathbb A$. We also
provide explicit multiplication formulas for cluster characters associated to
regular modules over the path algebra of any representation-infinite quiver.
We introduce quantized Chebyshev polynomials as deformations of generalized
Chebyshev polynomials previously introduced by the author in the context of
acyclic coefficient-free cluster algebras. We prove that these quantized
polynomials arise in cluster algebras with principal coefficients associated to
acyclic quivers of infinite representation types and equioriented Dynkin
quivers of type $\mathbb A$. We also study their interactions with bases and
especially canonically positive bases in affine cluster algebras.