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 A128204 #17 Feb 11 2024 12:38:44
%S A128204 0,1,4,9,2,11,22,35,20,3,22,43,66,41,14,43,12,45,10,47,8,49,6,51,98,
%T A128204 147,96,149,94,37,96,157,220,155,88,19,90,17,92,15,94,13,96,181,268,
%U A128204 179,270,177,82,179,80,181,78,183,76,185,74,187,72,189,70,191,68,193,320
%N A128204 a(0) = 0; a(n) = a(n-1) - (2n-1) if that number is positive and not already in the sequence, otherwise a(n) = a(n-1) + (2n-1).
%C A128204 'Recamán transform' (see A005132) of the odd numbers.
%H A128204 Nick Hobson, <a href="/A128204/a128204.py.txt">Python program for this sequence</a>
%H A128204 <a href="/index/Rea#Recaman">Index entries for sequences related to Recamán's sequence</a>
%e A128204 Consider n=7. We have a(6)=22 and try to subtract 13, the 7th odd number. The result, 9, is certainly positive, but we cannot use it because 9 is already in the sequence. So we must add 13 instead, getting a(7) = 22 + 13 = 35.
%o A128204 (PARI) A128204(N,s/*=1 to print all terms*/)={my(a=0,u=0); for( n=1,N, s&print1(a","); u=bitor(u,2^a+=if(a<2*n || bittest(u,a+1-2*n), 2*n-1,1-2*n)));a} \\ _M. F. Hasler_, Mar 07 2012
%Y A128204 Cf. A005132, A053461, A064365, A123483.
%K A128204 easy,nonn
%O A128204 0,3
%A A128204 _Nick Hobson_, Feb 19 2007