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 A019990 #18 Aug 02 2019 02:34:36 %S A019990 1,3,4,7,9,11,12,13,19,21,22,25,27,29,32,33,34,36,38,39,40,46,55,57, %T A019990 58,61,63,65,66,67,73,75,76,79,81,83,86,87,89,95,96,97,99,101,102,103, %U A019990 106,108,110,113,114,115,117,119,120,121,127,136,138,139,145 %N A019990 Indices n such that A307672(2*n) = 2. %C A019990 The original definition was: "A threequence, a 3-way partitioning of the integers: define a,b,c,A,B,C by a(0)=true, b(0)=c(0)=A(0)=B(0)=C(0)=false, a(n)=a(m) OR C(m) OR B(m), b(n)= b(m) OR A(m) OR C(m), c(n)= c(m) OR B(m) OR A(m), A(n)= A(m) OR b(m) OR c(m), B(n)= B(m) OR c(m) OR a(m), C(n)= C(m) OR a(m) OR b(m), where m = [ (n+1)/3 ]; sequence gives n such that b(2n) is true." - _Sean A. Irvine_, Apr 30 2019 %H A019990 R. W. Gosper, J. Z. Hunts, <a href="https://www.gathering4gardner.org/g4g10gift/math/Gosper_Bill-Strange_Series_for_Sierpinskis_Gasket.pdf">Strange Series for Sierpinski’s Gasket</a>, G4G10 Exchange Book, Vol. 2 (2012), 59-60. %Y A019990 Cf. A019989, A019991, A307672. %K A019990 nonn %O A019990 0,2 %A A019990 _Bill Gosper_ %E A019990 New name from _Sean A. Irvine_, Apr 30 2019