# Strange symmetry of Haldane model edge

In the Haldane model we break both the inversion symmetry and time reversal symmetry, as a consequence I didn't not have any expectations when it comes to symmetry of the energy bands. However, to my big surprise, the energy bands for the armchair edge is symmetric under the $$k$$ -> $$-k$$ exchange. How can we explain this symmetry and why are the symmetry of the two cases different?

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