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 A273543 #18 Dec 10 2016 19:44:18
%S A273543 16,18,20,24,27,28,32,35,36,40,44,45,48,52,54,55,56,60,63,64,65,68,70,
%T A273543 72,76,77,80,81,84,85,88,90,91,92,95,96,99,100,104,105,108,110,112,
%U A273543 115,116,117,119,120,124,126,128,130,132,133,135,136,140,143,144,145,148,152
%N A273543 Numbers for which 9 is a nontrivial quadratic residue.
%C A273543 Composite numbers greater than 9 may have additional solutions to x^2=9 (mod n) beyond the trivial 3^2 and (n-3)^2 solutions. Numbers may be squarefree, such as 35, 55, 65, 70.
%H A273543 Charles R Greathouse IV, <a href="/A273543/b273543.txt">Table of n, a(n) for n = 1..10000</a>
%F A273543 a(n) ~ n. More specifically, a(n) = n + 2n/log n + O(n/log^2 n). - _Charles R Greathouse IV_, Jun 08 2016
%F A273543 For n > 12, these are numbers not of the form k*p^e where k is in {1, 2, 3, 6}, p > 3 is prime, and e > 0. - _Charles R Greathouse IV_, Jun 08 2016
%e A273543 For 54, x^2 = 9 (mod 54) has nontrivial solutions 15, 21, 33, 39, and trivial solutions 3, 51. For 57 which is not in the list, x^2 = 9 (mod 57) has only the trivial solutions 3, 54.
%t A273543 Select[Range[10, 230], Length@PowerModList[9, 1/2, #] > 2 &]
%o A273543 (PARI) is(n)=for(k=4,n\2, if(k^2%n==9, return(1))); 0 \\ _Charles R Greathouse IV_, Jun 08 2016
%o A273543 (PARI) is(n)=if(n<16, return(0)); my(v2=valuation(n,2), v3=valuation(n,3), k=n/2^v2/3^v3); if(v2<3 && v3<2, if(v2>1,k>1,!isprimepower(k)), 1) \\ _Charles R Greathouse IV_, Jun 08 2016
%K A273543 nonn
%O A273543 1,1
%A A273543 _Dale Taylor_, May 25 2016