Quantum Heisenberg model on a sawtooth-chain lattice: rotation-invariant Green’s function methodTaras Hutak, Taras Krokhmalskii, Oleg Derzhko and Johannes RichterEur. Phys. J. B, 95 6 (2022) 93DOI: https://doi.org/10.1140/epjb/s10051-022-00359-0