Uniform chain
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H(k) = -2 t cos(k)
t = 1.000000

Dimerized chain
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t1 = t (1 - delta) = 0.750000
t2 = t (1 + delta) = 1.250000
delta = 0.250000

H(k) = [[0, t1 + t2 exp(-i k)],
        [t1 + t2 exp(+i k), 0]]

Uniform dispersion: E(k) = -2 t cos(k)
E_+(k), E_-(k) = +/- sqrt(t1^2 + t2^2 + 2 t1 t2 cos(k))
Band gap at k = pi: 2 |t1 - t2| = 1.000000