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 A138587 #15 Feb 18 2023 14:00:52
%S A138587 0,1,2,3,5,6,10,11,21,22,42,43,85,86,170,171,341,342,682,683,1365,
%T A138587 1366,2730,2731,5461,5462,10922,10923,21845,21846,43690,43691,87381,
%U A138587 87382,174762,174763,349525,349526,699050,699051,1398101,1398102,2796202,2796203,5592405
%N A138587 The union of all entries of A024495, A131708 and A024493 sorted into natural order.
%C A138587 The three sequences of the definition share the same special recurrence which reflects that each equals its own sequence of third differences.
%H A138587 G. C. Greubel, <a href="/A138587/b138587.txt">Table of n, a(n) for n = 0..1000</a>
%H A138587 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (-1,1,1,2,2).
%F A138587 a(n+8) == a(n) (mod 10), n > 1.
%F A138587 a(2*n+1) - a(2*n) = 1.
%F A138587 a(2*n) = A000975(n+1), n>0 (bisection).
%F A138587 From _R. J. Mathar_, Nov 22 2009: (Start)
%F A138587 a(n) = -a(n-1) +a(n-2) +a(n-3) +2*a(n-4) +2*a(n-5), n>6.
%F A138587 G.f.: x*(3*x+4*x^2+5*x^3+4*x^4+2*x^5+1)/((1+x)*(1-2*x^2)*(1+x^2)). (End)
%t A138587 CoefficientList[Series[x*(3*x + 4*x^2 + 5*x^3 + 4*x^4 + 2*x^5 + 1)/((1 + x)*(1 - 2*x^2)*(1 + x^2)), {x,0,50}], x] (* _G. C. Greubel_, Oct 03 2017 *)
%t A138587 LinearRecurrence[{-1,1,1,2,2},{0,1,2,3,5,6,10},50] (* _Harvey P. Dale_, Feb 18 2023 *)
%o A138587 (PARI) x='x+O('x^50); concat(0, Vec(x*(3*x+4*x^2+5*x^3+4*x^4 +2*x^5+ 1)/((1+x)*(1-2*x^2)*(1+x^2)))) \\ _G. C. Greubel_, Oct 03 2017
%K A138587 nonn
%O A138587 0,3
%A A138587 _Paul Curtz_, May 13 2008
%E A138587 Edited and extended by _R. J. Mathar_, Nov 22 2009