Isospectral reduction of the trimer Su-Schrieffer-Heeger lattice and its bulk-edge correspondence

The trimer Su-Schrieffer-Heeger (SSH3) lattice has attracted considerable attention due to its unique physical properties and rich phase diagram. However, the key symmetry responsible for the emergence of topological phases remains unclear. This study introduces an isospectral reduction model of the SSH3 lattice, revealing a bulk-edge correspondence enforced by hidden inversion symmetry. These findings provide a new strategy for exploring topological properties in systems beyond the tenfold way classification, while also paving the way for practical applications such as entangled-state transmission, controllable topological edge states, and enhanced pumping.