Xizhong Zheng Klaus Weihrauch The Arithmetical Hierarchy of Real Numbers. 51-65 2001 47 Math. Log. Q. 1 http://dx.doi.org/10.1002/1521-3870(200101)47:1<51::AID-MALQ51>3.0.CO;2-W db/journals/mlq/mlq47.html#ZhengW01