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 A215712 #20 Jun 02 2025 08:06:41
%S A215712 3,9,81,21,1359,2727,21837,21843,349515,699045,5592393,2796201,
%T A215712 89478471,178956963,1431655749,1431655761,22906492227,45812984481,
%U A215712 366503875905,22906492245,5864062014783,11728124029599,93824992236861,93824992236879,1501199875790139
%N A215712 Numerator of sum(i=1..n, 3*i/4^i ).
%C A215712 The limit as n goes to infinity is 4/3.
%D A215712 Calvin C. Clawson, The Beauty and Magic of Numbers. New York: Plenum Press (1996): 96.
%H A215712 Vincenzo Librandi, <a href="/A215712/b215712.txt">Table of n, a(n) for n = 1..1000</a>
%e A215712 a(4) = 21 because 3/4 + 6/16 + 9/64 + 12/256 = 3/4 + 3/8 + 9/64 + 3/64 = 48/64 + 24/64 + 9/64 + 3/64 = 84/64 = 21/16.
%t A215712 Table[Numerator[Sum[3i/4^i, {i, n}]], {n, 40}]
%o A215712 (Magma) [Numerator(&+[3*i/4^i: i in [1..n]]): n in [1..25]]; // _Bruno Berselli_, Sep 03 2012
%Y A215712 Cf. A215713 for the denominators.
%Y A215712 A036295/A036296 is the same with i/2^i instead of 3i/4^i.
%Y A215712 Cf. A122553.
%K A215712 nonn,easy,frac
%O A215712 1,1
%A A215712 _Alonso del Arte_, Aug 21 2012
%E A215712 a(17) corrected by _Vincenzo Librandi_, Sep 04 2012