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 A087446 #29 Dec 05 2018 20:18:02
%S A087446 1,6,16,21,31,36,46,51,61,66,76,81,91,96,106,111,121,126,136,141,151,
%T A087446 156,166,171,181,186,196,201,211,216,226,231,241,246,256,261,271,276,
%U A087446 286,291,301,306,316,321,331,336,346,351,361,366,376,381,391,396,406
%N A087446 Numbers that are congruent to {1, 6} mod 15.
%C A087446 3*a(n) is conjectured to be the number of edges (bonds) visited when walking around the boundary of a certain equilateral triangle construction at the n-th iteration. See the illustration in the link. Note that isthmus edges (bridges) are counted twice. The pattern is supposed to become the planar Archimedean net 3.12.12 when n -> infinity. - _Kival Ngaokrajang_, Nov 30 2014
%H A087446 Kival Ngaokrajang, <a href="/A087446/a087446.pdf">Illustration of initial terms</a>
%H A087446 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (1,1,-1)
%F A087446 G.f.: x*(1 + 5*x + 9*x^2)/((1 + x)*(1 - x)^2).
%F A087446 E.g.f.: (30*x-1)*exp(x)/4 + 5*exp(-x)/4.
%F A087446 a(n) = (18*n-1)/4 + 5*(-1)^n/4.
%F A087446 a(n) = 15*n - a(n-1) - 23, with a(1)=1. - _Vincenzo Librandi_, Aug 08 2010
%t A087446 #+{1,6}&/@(15*Range[0,30])//Flatten (* or *) LinearRecurrence[{1,1,-1},{1,6,16},60] (* _Harvey P. Dale_, Dec 05 2018 *)
%Y A087446 Cf. A001651, A047241, A087444, A087445.
%K A087446 nonn,easy
%O A087446 1,2
%A A087446 _Paul Barry_, Sep 04 2003
%E A087446 Editing: rewording of Kival Ngaokrajang's comment. - _Wolfdieter Lang_, Dec 06 2014