Guy, Richard K., Unsolved problems in number theory. Third edition, Problem Books in Mathematics, Springer, New York (2004), xviii+437 pp. Section C4 "Ulam numbers", p. 166: the U-numbers and five questions on them, introduced as some of those Recamán asked, among them positive density (marked as Ulam's), infinitely many consecutive pairs, and arbitrarily large gaps, with Muller's 20000 terms. Library home: Guy 2004.