Guy, Richard K., Unsolved problems in number theory. 3rd ed., Problem Books in Mathematics, Springer, New York (2004), xviii+437 pp. Section B33 "Largest divisor of a binomial coefficient", printed p. 135: "Erdős has also conjectured that for k>8, 2k is not the sum of distinct powers of 3 [28=35+32+3+1]", a stronger form of the page's question (no such power beyond 28, where the page asks only for finitely many), stated for its consequence that 3∣(2k2k+1) for k≥9. Library home: Guy 2004.