Mathematical analysis of whispering gallery modes in graded index optical micro-disk resonatorsStéphane Balac, Monique Dauge, Yannick Dumeige, Patrice Féron and Zoïs MoitierEur. Phys. J. D, 74 11 (2020) 221DOI: https://doi.org/10.1140/epjd/e2020-10303-5