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A.V. Malakhovskii, S.L. Gnatchenko, I.S. Kachur, V.G. Piryatinskaya, and I.A. Gudim, Low-temperature absorption spectra and electron structure of HoFe3(BO3)4 single crystal, Low Temp. Phys. 43, (2017) [Fiz. Nizk. Temp. 43, 764-771, (2017)] DOI: https://doi.org/10.1063/1.4985208.