Outline
I. Background: Friezes
II. Background: Cluster algebras
III. New results: Frieze vectors and unitary friezes
Part I. Friezes
A frieze is an image that repeats itself along one direction. The name comes from architecture, where a frieze is a decoration running horizontally below a ceiling or roof. From M. Ascher, Ethnomathematics, p. 162.
Conway - Coxeter frieze (1970s)
A (type ) frieze is an array such that
it is bounded above and below by a row of s
every diamond satisfies the diamond rule .
A Conway - Coxeter frieze consists of only positive integers.
Example (a Conway - Coxeter frieze)
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Children practicing arithmetic
Note: every frieze is completely determined by the 2nd row.
Children practicing arithmetic: Answer Key
What do the numbers around the integers count?
The corresponding frieze is below.
Theorem (Conway and Coxeter, 1970s)
A Conway - Coxeter frieze with nontrivial rows a triangulation of an -gon
Note: Hence Conway - Coxeter friezes are Catalan objects.
Part II. Cluster Algebra (Fomin - Zelevinsky, 2001)
Let be a quiver (a directed graph) on vertices with no loop and no 2-cycle.
E.g. Below is a quiver of affine Dynkin type , which has 1 arrow pointing counterclockwise and 2 arrows pointing clockwise.
A cluster algebra from is a subalgebra of the field of rational functions in variables.
The generators of are called cluster variables, which are computed from as we explain below.
Note: Python indexing starts at , but our indexing starts at .
Initial Seed (cluster + quiver)
Start with an initial cluster (a set of cluster variables) of size n.
Mutation
We can mutate a cluster at each of the vertices 1, 2, , n.
Below, we mutate the initial cluster at vertex to get a new cluster variable.
We get a new quiver by reversing all arrows adjacent to vertex .
Mutating all clusters
Continue mutating all clusters at all vertices.
Below, we mutate at after mutating at .
Laurent Phenomenon and Positivity
As we mutate many times, the cluster variables (which we are dividing by) get more and more complicated, but we keep producing positive Laurent polynomials.
Theorem (Fomin - Zelevinsky, Gross - Hacking, Sean Keel - Kontsevich, Lee - Schiffler):
Every cluster variable is a Laurent polynomial with positive coefficients in the initial cluster variables, that is, every cluster variable is where , that is, a polynomial with positive coefficients.
Part III. Frieze vectors and Unitary friezes
For the rest of the talk, we will discuss Frieze vectors and unitary friezes, joint with R. Schiffler.
Comments are welcome.
Type Frieze (over an integral domain)
In general, a frieze (of type ) is an array of elements of an integral domain such that
it is bounded above and below by a row of s
every diamond
satisfies the rule .
Example: a frieze over the cluster algebra
Note: I omit the rows of s.
Friezes over the integers
Specializing gives a Conway - Coxeter (positive integer) frieze
Specializing and gives
Frieze over the Gaussian integers
Specializing , , and gives
Frieze over the quadratic integer ring
Specializing , , gives
Friezes as ring homomorphisms
Given any quiver , let a frieze be a ring homomorphism where is an integral domain, for example .
Examples of friezes
defined by
for a cluster
Unitary friezes
A positive integral frieze is called unitary if can be obtained by specializing every element in one cluster to .
Theorem 1 (G, Schiffler)
Let be any quiver. The positive integral unitary friezes are in bijection with clusters.
Theorem 2 (G, Schiffler)
Every positive integral frieze of type is unitary.
Note:
An acyclic (no oriented cycles) quiver of type is a cyle with vertices with arrows oriented clockwise and arrows oriented counterclockwise.
Remark:
For type and , every positive integral frieze is unitary.
For type , , , and , there are non-unitary friezes.
Future directions
Type Dynkin affine
Classify the non-unitary friezes
Conjecture (based on Sage experiments): Up to cluster automorphisms (symmetry of the quiver), there are finitely many friezes.