“…Take x, y, and z to be the traces of the matrices ρ(a), ρ(b), and ρ(c) where a, b, and c are primitive elements of G with respective slopes 0/1, 1/0, and 1/1. Then, as shown in [9], the value of the specialization of U 1 (p/q) is the trace of ρ(g) where g is a primitive element of G with slope p/q. Example 5.4 (Markov numbers).…”