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M.Yu. Kagan, V.V. Val’kov, and P. Woelfle, Manifestation of the Upper Hubbard band in the 2D Hubbard model at low electron density: Низкоразмерные структуры, Low Temp. Phys. 37, (2011) [Fiz. Nizk. Temp. 37, 1046-1052, (2011)] DOI: https://doi.org/10.1063/1.3670026.