Quantum Heisenberg model on a sawtooth-chain lattice: rotation-invariant Green’s function method Taras Hutak, Taras Krokhmalskii, Oleg Derzhko and Johannes Richter Eur. Phys. J. B, 95 6 (2022) 93 Published online: 10 June 2022 DOI: 10.1140/epjb/s10051-022-00359-0