Nonnesting permutations avoiding 123

A nonnesting permutation of the multiset {1,1,2,2,…,n,n} is a word containing no subsequence abba with a distinct from b. Elizalde and Luo enumerated nonnesting permutations avoiding each set of two or more patterns of length three, and left open the enumeration of those avoiding the single pattern 123 (Problem 1 of their paper). We solve this problem. Writing $c_n$ for the number of nonnesting permutations of {1,1,…,n,n} avoiding 123 and $C(z)={\sum }_{n\ge 0}{c}_n{z}^n$, we prove $C(z)=1+z(1+U)\mathrm{/}(2-z(1+U)(1+V))$, where $V=1+z{V}^2$ and $U=1+3z+z{U}^2$ are the formal power series branches with constant term 1. In particular $C$ is algebraic of degree exactly 4 over $\mathbb{Q}(z)$, and $c_n\sim \frac{9+6\sqrt{3}}{8\sqrt{\pi }}{6}^n{n}^{-3\mathrm{/}2}$. The proof constructs a generating tree for these objects whose labels consist of two nonnegative integers, and applies the kernel method twice.