@article{ITA_2007__41_1_3_0,
     author = {Zheng, Xizhong},
     title = {On the hierarchies of $\Delta ^0_2$-real numbers},
     journal = {RAIRO - Theoretical Informatics and Applications - Informatique Th\'eorique et Applications},
     pages = {3--25},
     publisher = {EDP-Sciences},
     volume = {41},
     number = {1},
     year = {2007},
     doi = {10.1051/ita:2007008},
     mrnumber = {2330040},
     language = {en},
     url = {http://www.numdam.org/articles/10.1051/ita:2007008/}
}