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 A102603 #23 Jul 04 2025 19:21:58
%S A102603 21,45,69,93,117,141,165,189,213,237,261,285,309,333,357,381,405,429,
%T A102603 453,477,501,525,549,573,597,621,645,669,693,717,741,765,789,813,837,
%U A102603 861,885,909,933,957,981,1005,1029,1053,1077,1101,1125,1149,1173,1197,1221,1245
%N A102603 a(n) = 24*n + 21.
%H A102603 Tanya Khovanova, <a href="http://www.tanyakhovanova.com/RecursiveSequences/RecursiveSequences.html">Recursive Sequences</a>.
%H A102603 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (2,-1).
%F A102603 From _Elmo R. Oliveira_, Jul 01 2025: (Start)
%F A102603 G.f.: 3*(7 + x)/(1-x)^2.
%F A102603 E.g.f.: 3*(7 + 8*x)*exp(x).
%F A102603 a(n) = 3*A004771(n).
%F A102603 a(n) = 2*a(n-1) - a(n-2). (End)
%t A102603 Range[21, 2000, 24] (* _Vladimir Joseph Stephan Orlovsky_, Jun 14 2011 *)
%t A102603 24*Range[0,60]+21 (* or *) LinearRecurrence[{2,-1},{21,45},60] (* _Harvey P. Dale_, Oct 06 2015 *)
%o A102603 (PARI) a(n)=24*n+21 \\ _Charles R Greathouse IV_, Jun 14 2011
%o A102603 (PARI) Vec(3*(x+7)/(x-1)^2 + O(x^52)) \\ _Elmo R. Oliveira_, Jul 01 2025
%Y A102603 Cf. A004771.
%K A102603 easy,nonn
%O A102603 0,1
%A A102603 _Alexandre Wajnberg_, Jan 29 2005
%E A102603 More terms from _Elmo R. Oliveira_, Jul 01 2025