Su-Schrieffer-Heeger chain — the simplest 1D topological insulator with winding number and mid-gap edge states
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H = Σ(v·aₙ†bₙ + w·aₙ₊₁†bₙ + h.c.) — Winding number W = sign(w-v) for |w| ≠ |v|. Topological phase (W=1) when |w| > |v|: two zero-energy edge modes localized at the ends. A 1D prototype of bulk-boundary correspondence.