Cristian Calude Peter Hertling Bakhadyr Khoussainov Yongge Wang Recursively enumerable reals and Chaitin Omega numbers. 125-149 2001 255 Theor. Comput. Sci. 1-2 db/journals/tcs/tcs255.html#CaludeHKW01 http://dx.doi.org/10.1016/S0304-3975(99)00159-0