Guy, Richard K., Unsolved problems in number theory, third edition, Problem Books in Mathematics, Springer (2004), xviii+437 pp.; A19 "Values of $n$ making $n-2^k$ prime. Odd numbers not of the form $\pm p^a\pm2^b$.", printed p. 67: Erdős's conjecture that 4, 7, 15, 21, 45, 75 and 105 are the only such $n$, the verifications to $2^{44}$ ([MiWe69]) and $2^{77}$ (Uchiyama and Yorinaga), Vaughan's bound $x\exp(-(\ln x)^c)$ ([Va73]) and the conditional bounds $O(x^c)$, $c<1$, of Hooley and Narkiewicz. Library home: Guy 2004.

Mientka, Walter E. and Weitzenkamp, Roger C., On $f$-plentiful numbers. J. Combinatorial Theory (1969), 374-377.

Vaughan, R. C., Some applications of Montgomery's sieve. J. Number Theory (1973), 64-79.