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 A023691 #24 Mar 06 2025 16:02:56
%S A023691 511,767,895,959,991,1007,1015,1019,1021,1022,1279,1407,1471,1503,
%T A023691 1519,1527,1531,1533,1534,1663,1727,1759,1775,1783,1787,1789,1790,
%U A023691 1855,1887,1903,1911,1915,1917,1918,1951,1967,1975
%N A023691 Numbers with exactly 9 ones in binary expansion.
%H A023691 Ivan Neretin, <a href="/A023691/b023691.txt">Table of n, a(n) for n = 1..10000</a>
%H A023691 Robert Baillie, <a href="http://arxiv.org/abs/0806.4410">Summing the curious series of Kempner and Irwin</a>, arXiv:0806.4410 [math.CA], 2008-2015. See p. 18 for Mathematica code irwinSums.m.
%F A023691 a(n+1) = A057168(a(n)). - _M. F. Hasler_, Aug 27 2014
%F A023691 Sum_{n>=1} 1/a(n) = 1.386348105265697723853732153017686670695581836109569165990080192653647019078... (calculated using Baillie's irwinSums.m, see Links). - _Amiram Eldar_, Feb 14 2022
%t A023691 Select[ Range[ 511, 2048 ], (Count[ IntegerDigits[ #, 2 ], 1 ]==9)& ]
%o A023691 (PARI) is_A023691(n)=hammingweight(n)==9 \\ _M. F. Hasler_, Aug 27 2014
%o A023691 (PARI) print1(t=2^9-1); for(i=2, 50, print1(", "t=A057168(t))) \\ _M. F. Hasler_, Aug 27 2014
%o A023691 (Python)
%o A023691 from itertools import islice
%o A023691 def A023691_gen(): # generator of terms
%o A023691 yield (n:=511)
%o A023691 while True: yield (n:=((n&~(b:=n+(a:=n&-n)))>>a.bit_length())^b)
%o A023691 A023691_list = list(islice(A023691_gen(),30)) # _Chai Wah Wu_, Mar 06 2025
%Y A023691 Cf. A000079, A018900, A014311, A014312, A014313, A023688, A023689, A023690 (Hamming weight = 1, 2, ..., 8), A057168.
%K A023691 nonn,base,easy
%O A023691 1,1
%A A023691 _Olivier Gérard_