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 A066489 #8 Oct 22 2023 17:53:22
%S A066489 10,1010,1100,100110,101010,110100,111000,10001110,10010110,10101010,
%T A066489 10110010,11001100,11010100,11101000,11110000,1000011110,1000101110,
%U A066489 1001010110,1001100110,1010011010,1010101010,1011010010,1011100010,1100011100,1100101100,1101010100
%N A066489 Binary expansion of n followed by its reverse complement.
%e A066489 a(2) = 1010 because 2 in binary is 10, the complement of which is 01, the reverse of which is 10, hence (10)(10). - _Sean A. Irvine_, Oct 22 2023
%p A066489 a:= n-> (l-> parse(cat(seq(l[-i], i=1..nops(l)), 1-~l[])))(Bits[Split](n)):
%p A066489 seq(a(n), n=1..30); # _Alois P. Heinz_, Oct 22 2023
%o A066489 (PARI) a(n)={my(v=binary(n)); fromdigits(concat(v,vector(#v,i,1-v[#v+1-i])))} \\ _Andrew Howroyd_, Oct 22 2023
%Y A066489 Binary expansion of numbers in A035928.
%K A066489 base,easy,nonn
%O A066489 1,1
%A A066489 _John McNamara_, Jan 09 2002
%E A066489 Title clarified and more terms from _Sean A. Irvine_, Oct 22 2023