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 A115639 #6 Sep 08 2022 08:45:24
%S A115639 1,1,4,4,4,4,16,16,16,16,16,16,16,16,64,64,64,64,64,64,64,64,64,64,64,
%T A115639 64,64,64,64,64,256,256,256,256,256,256,256,256,256,256,256,256,256,
%U A115639 256,256,256,256,256,256,256,256,256,256,256,256,256,256,256,256,256,256
%N A115639 First column of divide-and-conquer triangle A115636.
%H A115639 G. C. Greubel, <a href="/A115639/b115639.txt">Table of n, a(n) for n = 0..1000</a>
%F A115639 G.f.: (1/(1-x))*( 1/4 + (3/4)*Sum_{k>=1} 4^(k-1)*x^(2^k-2) ).
%F A115639 a(n) = 4^(floor(log_2(n+2)) - 1). - _G. C. Greubel_, Nov 23 2021
%t A115639 4^(Floor[Log[2, Range[0, 80] +2]] -1) (* _G. C. Greubel_, Nov 23 2021 *)
%o A115639 (Magma) [4^(Ilog2(n+2) -1) : n in [0..80] ]; // _G. C. Greubel_, Nov 23 2021
%o A115639 (Sage) [4^(floor(log(n+2, 2)) -1) for n in (0..80)] # _G. C. Greubel_, Nov 23 2021
%Y A115639 Cf. A000523, A115636.
%K A115639 easy,nonn
%O A115639 0,3
%A A115639 _Paul Barry_, Jan 27 2006