Guy, Richard K., Unsolved problems in number theory. Third edition, Problem Books in Mathematics, Springer (2004), xviii+437 pp. Section A8 "Gaps between primes. Twin primes.", printed pp. 33--34: Cramér's conditional bound ∑n<xdn2<cx(lnx)4, and "Erdős conjectures that the right-hand side should be cx(lnx)2, but thinks that there is no hope of a proof". Library home: Guy 2004.