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 A131721 #17 Mar 01 2024 07:25:25
%S A131721 1,1,3,7,17,41,100,240,580,1400,3380,8160,19700,47560,114820,277200,
%T A131721 669220,1615640,3900500,9416640,22733780,54884200,132502180,319888560,
%U A131721 772279300,1864447160,4501173620,10866794400,26234762420,63336319240
%N A131721 Overlay of Pell companion numbers: a(n) = A001333(n) + A001333(n-6).
%H A131721 Paolo Xausa, <a href="/A131721/b131721.txt">Table of n, a(n) for n = 0..1000</a>
%H A131721 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (2,1).
%F A131721 a(n) = A001333(n), n<6. a(n) = A001333(n)+A001333(n-6) = 20*A000129(n-3), n>=6.
%F A131721 O.g.f.: (x-1)(x^2+1)(x^4-x^2+1)/(-1+2x+x^2). - _R. J. Mathar_, Jul 16 2008
%F A131721 For n>5, a(n) = 100*A000129(n+1) -240*A000129(n). - _R. J. Mathar_, Feb 05 2024
%t A131721 LinearRecurrence[{2, 1}, {1, 1, 3, 7, 17, 41, 100, 240}, 50] (* _Paolo Xausa_, Mar 01 2024 *)
%Y A131721 Cf. A131710.
%K A131721 nonn,easy
%O A131721 0,3
%A A131721 _Paul Curtz_, Sep 15 2007
%E A131721 Edited and extended by _R. J. Mathar_, Jul 16 2008