Axes are kₓa and k_yb, so circles in k look like ellipses for graphene. A solution ψn = ψ₀eik·rn turns the matrix equation into E(k) = Σm Hnmeik·(rm−rn) (Eq. 17.18). For graphene, a lattice with a two-atom basis, it becomes a 2×2 matrix h(k) with eigenvalues ε ± |h₀(k)| (Eqs. 17.22–17.26). Click on the k-map to pick a k-point.