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 A039721 #30 Jul 02 2025 16:01:56
%S A039721 1,2,4,12,32,96,280,840,2496,7488,22400,67200,201408,604224,1812112,
%T A039721 5436336,16307328,48921984,146760960,440282880,1320833664,3962500992,
%U A039721 11887458176,35662374528,106986989184,320960967552,962882499840,2888647499520,8665941290112
%N A039721 a(1) = 1, a(m+1) = 2*Sum_{k=1..floor((m+1)/2)} a(m+1-k).
%H A039721 Robert Israel, <a href="/A039721/b039721.txt">Table of n, a(n) for n = 1..212</a>
%F A039721 a(1)=1, a(2)=2, a(2m+1)=3*a(2m)-2*a(m), a(2m+2)=3*a(2m+1) (m is positive integer).
%e A039721 a(6)=2*(a(5)+a(4)+a(3)) = 2*(32+12+4) = 96.
%p A039721 a[1]:= 1;
%p A039721 for m from 1 to 100 do
%p A039721 a[m+1]:= 2*add(a[m+1-k],k=1..floor((m+1)/2));
%p A039721 od:
%p A039721 seq(a[i],i=1..100); # _Robert Israel_, May 18 2014
%t A039721 Fold[Append[#1, 2 Total[#1[[#2 - Range[Floor[#2/2] ] ]] ] ] &, {1}, Range[2, 29]] (* _Michael De Vlieger_, Dec 11 2017 *)
%o A039721 (PARI) lista(nn) = {v = vector(nn); v[1] = 1; for (n=2, nn, v[n] = 2*sum(k=1, n\2, v[n-k]);); v;} \\ _Michel Marcus_, May 18 2014
%Y A039721 Cf. A039722 (similar definition).
%K A039721 easy,nonn
%O A039721 1,2
%A A039721 _Leroy Quet_, Dec 11 1999
%E A039721 More terms from _James Sellers_, May 04 2000
%E A039721 Two more terms from _Michel Marcus_, May 18 2014