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 A216852 #34 Sep 08 2022 08:46:03
%S A216852 9,9,-9,9,9,45,63,117,153,225,279,369,441,549,639,765,873,1017,1143,
%T A216852 1305,1449,1629,1791,1989,2169,2385,2583,2817,3033,3285,3519,3789,
%U A216852 4041,4329,4599,4905,5193,5517,5823,6165,6489,6849,7191,7569,7929,8325,8703
%N A216852 18k^2-36k+9 interleaved with 18k^2-18k+9 for k>=0.
%C A216852 The sequence is present as a family of single interleaved sequence of which there are many which are separated or factored out to give individual sequences. The larger sequence produces two smaller interleaved sequences where one of them has the formulas above and the other interleaved sequence has the formulas (18n^2-24n+1) and (18n^2-6n+5). The latter interleaved sequence is A214493. There are three sequences in this family.
%H A216852 Eddie Gutierrez <a href="http://www.oddwheel.com/square_sequencesII.html">New Interleaved Sequences Part B</a> on oddwheel.com, Section B1 Line No. 22 (square_sequencesII.html) Part B
%H A216852 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (2, 0, -2, 1).
%F A216852 From _Bruno Berselli_, Oct 01 2012: (Start)
%F A216852 G.f.: 9*(1-x-3*x^2+5*x^3)/((1+x)*(1-x)^3).
%F A216852 a(n) = (9/4)*(2*n*(n-4)-3*(-1)^n+7).
%F A216852 a(n) = 9*A178218(n-3) with A178218(-3)=1, A178218(-2)=1, A178218(-1)=-1, A178218(0)=1. (End)
%F A216852 a(0)=9, a(1)=9, a(2)=-9, a(3)=9, a(n)=2*a(n-1)-2*a(n-3)+a(n-4). - _Harvey P. Dale_, Apr 26 2014
%t A216852 Flatten[Table[{18 n^2 - 36 n + 9, 18 n^2 - 18 n + 9}, {n, 0, 23}]] (* _Bruno Berselli_, Oct 01 2012 *)
%t A216852 Flatten[Table[18n^2+9-{36n,18n},{n,0,50}]] (* or *) LinearRecurrence[ {2,0,-2,1},{9,9,-9,9},100] (* _Harvey P. Dale_, Apr 26 2014 *)
%o A216852 (Magma) &cat[[18*k^2-36*k+9, 18*k^2-18*k+9]: k in [0..23]]; // _Bruno Berselli_, Oct 01 2012
%o A216852 (PARI) vector(47, n, k=(n-1)\2; if(n%2, 18*k^2-36*k+9, 18*k^2-18*k+9)) \\ _Bruno Berselli_, Oct 01 2012
%Y A216852 Cf. A178218, A214345, A214393, A214405, A216844, A216865, A216875, A216876.
%K A216852 sign,easy
%O A216852 0,1
%A A216852 _Eddie Gutierrez_, Sep 17 2012
%E A216852 Definition rewritten by _Bruno Berselli_, Oct 25 2012