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 A230780 #47 Jul 11 2020 02:31:44
%S A230780 1,2,3,4,5,6,8,9,10,11,12,15,16,17,18,20,22,23,24,25,27,29,30,32,33,
%T A230780 34,36,40,41,44,45,46,47,48,50,51,53,54,55,58,59,60,64,66,68,69,71,72,
%U A230780 75,80,81,82,83,85,87,88,89,90,92,94,96,99,100,101,102,106,107
%N A230780 Positive numbers without a prime factor congruent to 1 (mod 6).
%C A230780 The sequence is closed under multiplication. Primitive elements are 3 and the primes of form 3*k+2.
%C A230780 a(n)^2 is not expressible as x^2+xy+y^2 with x and y positive integers.
%C A230780 Analog of A004144 (nonhypotenuse numbers) for 120-degree angle triangles: a(n) is not the length of the longest side of such a triangle with integer sides.
%C A230780 It might have been natural to include 0 in this sequence. - _M. F. Hasler_, Mar 04 2018
%H A230780 Ray Chandler, <a href="/A230780/b230780.txt">Table of n, a(n) for n = 1..10000</a> (first 254 terms from Jean-Christophe Hervé)
%H A230780 F. Javier de Vega, <a href="https://arxiv.org/abs/2003.13378">An extension of Furstenberg's theorem of the infinitude of primes</a>, arXiv:2003.13378 [math.NT], 2020.
%H A230780 August Lösch, <a href="http://archive.org/stream/economicsoflocat00ls#page/116/mode/2up">Economics of Location</a> (1954), see pp. 117f.
%H A230780 U. P. Nair, <a href="https://arxiv.org/abs/math/0408107">Elementary results on the binary quadratic form a^2+ab+b^2</a>, arXiv:math/0408107 [math.NT], 2004.
%H A230780 <a href="/index/Aa#A2">Index entries for sequences related to A2 = hexagonal = triangular lattice</a>
%F A230780 A005088(a(n)) = 0.
%t A230780 Join[{1}, Select[Range[2, 110], ! MemberQ[Union[Mod[Transpose[ FactorInteger[#]][[1]], 6]], 1] &]] (* _T. D. Noe_, Nov 24 2013 *)
%t A230780 Join[{1},Select[Range[110],NoneTrue[FactorInteger[#][[All,1]],Mod[#,6] == 1&]&]] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Feb 03 2019 *)
%o A230780 (Haskell)
%o A230780 a230780 n = a230780_list !! (n-1)
%o A230780 a230780_list = filter (all (/= 1) . map (flip mod 6) . a027748_row) [1..]
%o A230780 -- _Reinhard Zumkeller_, Apr 09 2014
%o A230780 (PARI) is_A230780(n)=!setsearch(Set(factor(n)[,1]%6),1) \\ _M. F. Hasler_, Mar 04 2018
%Y A230780 Cf. A002476, A005088, complement of A050931.
%Y A230780 Cf. A004144 (analog for 4k+1 primes and right triangles).
%Y A230780 Cf. A027748.
%K A230780 nonn
%O A230780 1,2
%A A230780 _Jean-Christophe Hervé_, Nov 23 2013