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 A121684 #14 May 10 2022 10:28:49
%S A121684 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,17,18,19,20,21,22,23,26,28,29,30,
%T A121684 31,33,34,35,37,38,39,41,42,43,44,46,47,51,52,53,55,57,58,59,60,61,62,
%U A121684 65,66,67,68,69,70,71,73,74,76,77,78,79,82,83,84,85,86,87,89,91,92,93,94,95,97
%N A121684 Union of {8, 9, 18}, S, 2S and 4S, where S = odd squarefree numbers (A056911).
%C A121684 Numbers n such that the cyclic group Z_n is a CI-group.
%C A121684 The asymptotic density of this sequence is 7/Pi^2. - _Amiram Eldar_, May 10 2022
%H A121684 Amiram Eldar, <a href="/A121684/b121684.txt">Table of n, a(n) for n = 1..10000</a>
%H A121684 B. Alspach and M. Mishna, <a href="http://dx.doi.org/10.1016/S0012-365X(02)00319-9">Enumeration of Cayley graphs and digraphs</a>, Discr. Math., 256 (2002), 527-539.
%H A121684 M. Mishna, <a href="http://people.math.sfu.ca/~mmishna/">Home Page</a>
%H A121684 M. Muzychuk, <a href="http://dx.doi.org/10.1016/S0012-365X(97)81804-3">On Adam's conjecture for circulant graphs</a>, Discr. Math. 167 (1997), 497-510.
%t A121684 With[{upto=100},Select[Sort[Join[{8,9,18},Flatten[{#,2#,4#}&/@ Select[ Range[1,upto,2],SquareFreeQ]]]],#<=upto&]] (* _Harvey P. Dale_, Sep 03 2015 *)
%Y A121684 Cf. A056911, A078779, A121176.
%K A121684 nonn
%O A121684 1,2
%A A121684 _N. J. A. Sloane_, Sep 13 2006