 The following construction is described by Kushner (1984).Let "A" be any recursively enumerable set of natural numbers that is not decidable, and let ("a""i") be a computable enumeration of "A" without repetition.Define a sequence ("q""n") of rational numbers with the rule
It is clear that each "q""n" is nonnegative and rational, and that the sequence "q""n" is strictly increasing.