http://oeis.org/A001370 It has been pointed out on MathFun that the comment "a(n) ~ n*e/2 where e is the base of the natural logarithm 2,7182…", while not technically incorrect, should in fact state: a(n) ~ n*9*log10(2)/2. It just happens that 9*log10(2) is coincidentally close to e.