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 A133090 #17 Apr 04 2024 14:42:15
%S A133090 1,1,5,3,9,5,13,7,17,9,21,11,25,13,29,15,33,17,37,19,41,21,45,23,49,
%T A133090 25,53,27,57,29,61,31,65,33,69,35,73,37,77,39,81,41,85,43,89,45,93,47,
%U A133090 97,49,101,51,105,53,109,55,113,57,117,59,121,61,125,63,129,65,133,67,137,69,141,71
%N A133090 A133081 * [1,2,3,...].
%H A133090 G. C. Greubel, <a href="/A133090/b133090.txt">Table of n, a(n) for n = 1..5000</a>
%H A133090 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (0,2,0,-1).
%F A133090 2k+1 interleaved with 4k+1.
%F A133090 a(n) = (3/2)*n - (n/2)*(-1)^n - 1. - _Ridouane Oudra_, Sep 08 2023
%F A133090 From _Stefano Spezia_, Dec 04 2023: (Start)
%F A133090 O.g.f.: x*(1 + x + 3*x^2 + x^3)/((1 - x)^2*(1 + x)^2).
%F A133090 E.g.f.: 1 + (2*x - 1)*cosh(x) + (x - 1)*sinh(x). (End)
%p A133090 seq((3/2)*n - (n/2)*(-1)^n - 1, n=1..60); # _Ridouane Oudra_, Sep 08 2023
%t A133090 Table[Riffle[{4*n + 1}, {2*n + 1} ], {n, 0, 50}] // Flatten (* _G. C. Greubel_, Oct 21 2017 *)
%o A133090 (Python)
%o A133090 def A133090(n): return (n<<1)-1 if n&1 else n-1 # _Chai Wah Wu_, Apr 04 2024
%Y A133090 Cf. A133081.
%K A133090 nonn,easy
%O A133090 1,3
%A A133090 _Gary W. Adamson_, Sep 09 2007
%E A133090 Terms a(17) onward added by _G. C. Greubel_, Oct 21 2017