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 A026476 #42 Apr 15 2025 12:58:36
%S A026476 1,3,4,9,12,23,26,37,40,51,54,65,68,79,82,93,96,107,110,121,124,135,
%T A026476 138,149,152,163,166,177,180,191,194,205,208,219,222,233,236,247,250,
%U A026476 261,264,275,278,289,292,303,306,317,320,331,334,345,348,359,362,373,376
%N A026476 For n>3, a(n) = 7*n - 21 + 2*(-1)^n.
%C A026476 The old definition of this sequence was "a(n) = least positive integer > a(n-1) and not equal to a(i)+a(j)+a(k) for 1<=i<=j<=k<=n". However, Ralf Stephan observes that this does not fit the terms shown. (It produces A109474.) The present definition (due to Stephan) has been adopted as a temporary solution. It would be nice to have a definition similar to the original one. - _N. J. A. Sloane_, Nov 24 2004
%C A026476 From _Philippe Deléham_, Nov 21 2016: (Start)
%C A026476 First differences are 2, 1, 5, 3, 11, 3, 11, 3, 11, 3, 11, 3, 11, 3, 11,...
%C A026476 For n>3, numbers that are congruent to 9 or 12 mod 14. (End)
%H A026476 Vincenzo Librandi, <a href="/A026476/b026476.txt">Table of n, a(n) for n = 1..1000</a>
%H A026476 Yaohui Zhu, Kaiming Sun, Zhengdong Luo, and Lingfeng Wang, <a href="https://doi.org/10.1609/aaai.v39i2.32162">Progressive Self-Learning for Domain Adaptation on Symbolic Regression of Integer Sequences</a>, Proc. 39th AAAI Conf. Artif. Intel. (2025) Vol. 39, No. 1, 1692-1699. See p. 1698.
%H A026476 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (1,1,-1).
%F A026476 a(n) = a(n-1)+a(n-2)-a(n-3) for n>6. G.f.: x*(1+2*x+3*x^3+2*x^4+6*x^5)/((1-x)^2*(1+x)). - _Colin Barker_, May 02 2012
%t A026476 CoefficientList[Series[(1 + 2 x + 3 x^3 + 2 x^4 + 6 x^5)/((1 - x)^2 (1 + x)), {x, 0, 60}], x] (* _Vincenzo Librandi_, Oct 18 2013 *)
%o A026476 (Magma) [1,3,4] cat [7*n - 21 + 2*(-1)^n: n in [4..60]]; // _Vincenzo Librandi_, Oct 18 2013
%o A026476 (PARI) a(n) = if(n>3, 7*n - 21 + 2*(-1)^n, [1,3,4][n]) \\ _Charles R Greathouse IV_, Nov 21 2016
%Y A026476 Cf. A026472, A026474, A109474.
%K A026476 nonn,easy
%O A026476 1,2
%A A026476 _Clark Kimberling_
%E A026476 More terms from David Duran (dduran(AT)ashland.edu), Dec 14 2005