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 A169726 #30 Aug 16 2025 07:04:38
%S A169726 1,7,37,169,721,2977,12097,48769,195841,784897,3142657,12576769,
%T A169726 50319361,201302017,805257217,3221127169,12884705281,51539214337,
%U A169726 206157643777,824632147969,3298531737601,13194133241857,52776545550337,211106207367169,844424879800321
%N A169726 a(n) = 3*2^n*(2^n-1) + 1.
%C A169726 A subsequence of the centered hexagonal numbers A003215.
%H A169726 Vincenzo Librandi, <a href="/A169726/b169726.txt">Table of n, a(n) for n = 0..1000</a>
%H A169726 Alice V. Kleeva, <a href="/A169726/a169726a.jpg">Grid for this sequence</a>
%H A169726 Alice V. Kleeva, <a href="/A169726/a169726b.jpg">Illustration of initial terms</a>
%H A169726 Robert Munafo, <a href="http://www.mrob.com/pub/math/seq-a169720.html">Sequence A169720, and two others by Alice V. Kleeva</a>
%H A169726 Robert Munafo, <a href="/A169720/a169720.pdf">Sequence A169720, and two others by Alice V. Kleeva</a> [Cached copy, in pdf format, included with permission]
%H A169726 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (7,-14,8). - _R. J. Mathar_, Apr 26 2010
%F A169726 From _R. J. Mathar_, Apr 26 2010: (Start)
%F A169726 a(n) = 7*a(n-1) - 14*a(n-2) + 8*a(n-3).
%F A169726 G.f.: ( -1-2*x^2 ) / ( (x-1)*(2*x-1)*(4*x-1) ). (End)
%F A169726 a(n) = (2*A169720(n)+1)/3. - _L. Edson Jeffery_, Dec 03 2012
%p A169726 A169726:=n->3*2^n*(2^n-1)+1: seq(A169726(n), n=0..30); # _Wesley Ivan Hurt_, Sep 14 2014
%t A169726 CoefficientList[Series[(-1 - 2*x^2)/((x-1)*(2*x-1)*(4*x-1)), {x, 0, 30}],x] (* _Vincenzo Librandi_, Dec 04 2012 *)
%t A169726 Table[c=2^n;3c(c-1)+1,{n,0,30}] (* or *) LinearRecurrence[{7,-14,8},{1,7,37},30] (* _Harvey P. Dale_, Nov 22 2013 *)
%o A169726 (Magma) I:=[1, 7, 37]; [n le 3 select I[n] else 7*Self(n-1) -14*Self(n-2) +8*Self(n-3): n in [1..30]]; // _Vincenzo Librandi_, Dec 04 2012
%o A169726 (Magma) [3*2^n*(2^n-1)+1 : n in [0..30]]; // _Wesley Ivan Hurt_, Sep 14 2014
%Y A169726 Cf. A000079, A003215, A169720-A169727.
%K A169726 nonn,easy
%O A169726 0,2
%A A169726 Alice V. Kleeva (alice27353(AT)gmail.com), Jan 19 2010