You can find the proof on the arxiv:
Matt DeVos, Luis Goddyn, Bojan Mohar, Robert Samal: Cayley sum graphs and eigenvalues of -fullerenes
Robert Samal
Link to preprint
You can find the proof on the arxiv:
Matt DeVos, Luis Goddyn, Bojan Mohar, Robert Samal: Cayley sum graphs and eigenvalues of -fullerenes
Robert Samal