This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A139817 #10 Apr 05 2015 09:42:11
%S A139817 0,1,3,7,14,30,62,125,253,509,1020,2044,4092,8188,16379,32763,65531,
%T A139817 131066,262138,524282,1048569,2097145,4194297,8388601,16777208,
%U A139817 33554424,67108856,134217719,268435447,536870903,1073741814,2147483638,4294967286,8589934582
%N A139817 2^n - number of digits of 2^n.
%F A139817 a(n) = A000079(n) - A055642(A000079(n)). - _Michel Marcus_, Jul 18 2013
%t A139817 Table[2^n-IntegerLength[2^n],{n,0,40}] (* _Harvey P. Dale_, Apr 05 2015 *)
%o A139817 (PARI) a(n) = 2^n - length(digits(2^n)) \\ _Michel Marcus_, Jul 18 2013
%Y A139817 Cf. A000079, A055642.
%K A139817 nonn,base
%O A139817 0,3
%A A139817 _Rolf Pleisch_, May 23 2008
%E A139817 More terms from _N. J. A. Sloane_, Jun 29 2008