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The Colored Hofstadter Butterfly for the Honeycomb Lattice

We rely on a recent method for determining edge spectra and we use it to compute the Chern numbers for Hofstadter models on the honeycomb lattice having rational magnetic flux per unit cell. Based on the bulk-edge correspondence, the Chern number $σ_H$ is given as the winding number of an eigenvector of a $2 \times 2$ transfer matrix, as a function of the quasi-momentum $k \in (0,2 π)$. This method is computationally efficient (of order $O(n^4)$ in the resolution of the desired image). It also shows that for the honeycomb lattice the solution for $σ_H $ for flux $p/q$ in the $r$-th gap conforms with the Diophantine equation $r=σ_H\cdot p+ s\cdot q$, which determines $σ_H \mod q$. A window such as $σ_H \in(-q/2,q/2)$, or possibly shifted, provides a natural further condition for $σ_H$, which however turns out not to be met. Based on extensive numerical calculations, we conjecture that the solution conforms with the relaxed condition $σ_H\in(-q,q)$.