Perturbation expansion, Bogoliubov inequality and integral representations in nonextensive Tsallis statisticsR. S. Mendes, S. F. Kwok, E. K. Lenzi and J. N. MakiEur. Phys. J. B, 10 2 (1999) 353-359DOI: https://doi.org/10.1007/s100510050864