On the perturbation theory in spatially closed background Ali A. Asgari, Amir H. Abbassi and Jafar Khodagholizadeh Eur. Phys. J. C, 74 6 (2014) 2917 Published online: 04 June 2014 DOI: 10.1140/epjc/s10052-014-2917-0