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 A028742 #12 Nov 29 2019 01:36:04 %S A028742 2,3,8,10,11,12,14,15,17,18,19,21,26,27 %N A028742 Nonsquares mod 29. %e A028742 Since 12 is not a perfect square and there are no solutions to x^2 = 12 mod 29, 12 is in the sequence. %e A028742 Although 13 is not a perfect square either, there are solutions to x^2 = 13 mod 29, such as x = 10, x = 19, so 13 is not in the sequence. %t A028742 Complement[Range[0, 28], PowerMod[Range[29], 2, 29]] (* _Alonso del Arte_, Nov 29 2019 *) %o A028742 (Scala) val squaresMod29 = (1 to 29).map(n => n * n).map(_ % 29) %o A028742 (0 to 28).diff(squaresMod29) // _Alonso del Arte_, Nov 29 2019 %Y A028742 Cf. A010391. %K A028742 nonn,fini,full %O A028742 1,1 %A A028742 _N. J. A. Sloane_