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 A008602 #52 Apr 10 2025 13:54:11
%S A008602 0,20,40,60,80,100,120,140,160,180,200,220,240,260,280,300,320,340,
%T A008602 360,380,400,420,440,460,480,500,520,540,560,580,600,620,640,660,680,
%U A008602 700,720,740,760,780,800,820,840,860,880,900,920,940,960,980,1000
%N A008602 Multiples of 20.
%C A008602 The multiples of 20 are exactly those integers which do not have a multiple whose decimal digits are of alternating parity. (International Mathematical Olympiad 2004, problem 6, see A110303) - _Joseph Myers_, Jul 13 2004
%H A008602 Vincenzo Librandi, <a href="/A008602/b008602.txt">Table of n, a(n) for n = 0..1000</a>
%H A008602 Tanya Khovanova, <a href="http://www.tanyakhovanova.com/RecursiveSequences/RecursiveSequences.html">Recursive Sequences</a>.
%H A008602 INRIA Algorithms Project, <a href="http://ecs.inria.fr/services/structure?nbr=332">Encyclopedia of Combinatorial Structures 332</a>.
%H A008602 Luis Manuel Rivera, <a href="http://arxiv.org/abs/1406.3081">Integer sequences and k-commuting permutations</a>, arXiv preprint arXiv:1406.3081 [math.CO], 2014-2015.
%H A008602 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (2,-1).
%F A008602 G.f.: 20*x/(x - 1)^2. - _Vincenzo Librandi_, Jun 10 2013
%F A008602 E.g.f.: 20*x*exp(x). - _Stefano Spezia_, Feb 20 2020
%F A008602 From _Elmo R. Oliveira_, Apr 10 2025: (Start)
%F A008602 a(n) = 20*n = 2*A008592(n) = 10*A005843(n) = A317095(n)/2.
%F A008602 a(n) = 2*a(n-1) - a(n-2). (End)
%t A008602 Range[0, 1500, 20] (* _Vladimir Joseph Stephan Orlovsky_, Jun 01 2011 *)
%t A008602 CoefficientList[Series[20 x / (x - 1)^2, {x, 0, 60}], x] (* _Vincenzo Librandi_, Jun 10 2013 *)
%o A008602 (PARI) a(n)=20*n \\ _Charles R Greathouse IV_, Sep 28 2015
%o A008602 (Scala) (0 to 1000 by 20).toList // _Alonso del Arte_, Feb 20 2020
%Y A008602 Cf. A005843, A008592, A008600, A008601, A110303 (complement), A215145, A317095.
%K A008602 nonn,easy
%O A008602 0,2
%A A008602 _N. J. A. Sloane_