Bosonic particle pair of the Creutz ladder become itinerant due to particle-particle interaction although its one particle spectrum is completely flat implying localized in real space. Then modulation of the interaction induces gap of the effective hamiltonian of the pairs (doublons). With suitable choice of the boudary, in-gap modes of the doublons appear associated with the topological nature of the effective bulk. It’s a new class of the bulk-edge correspondence as for the original system. The paper, “Interaction-induced doublons and embedded topological subspace in a complete flat-band system” by Yoshihito Kuno, Tomonari Mizoguchi, and Yasuhiro Hatsugai, has been published in Phys. Rev. A. Have a look at. Also see arXiv:2009.08134.