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 A028726 #9 Dec 13 2019 14:03:15 %S A028726 2,5,6,7,8,11 %N A028726 Nonsquares mod 13. %e A028726 Since 2 is not a perfect square and there are no solutions to x^2 = 2 mod 13, 2 is in the sequence. %e A028726 Although 3 is not a perfect square either, there are solutions to x^2 = 3 mod 13, such as x = 4, x = 9. Hence 3 is not in the sequence. %t A028726 Select[Range[0, 12], JacobiSymbol[#, 13] == -1 &] (* _Alonso del Arte_, Dec 13 2019 *) %o A028726 (Scala) (0 to 12).diff((1 to 13).map(n => (n * n) % 13)) // _Alonso del Arte_, Dec 13 2019 %Y A028726 Cf. A010376. %K A028726 nonn,fini,full %O A028726 1,1 %A A028726 _N. J. A. Sloane_