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 A028733 #9 Dec 20 2019 20:45:09 %S A028733 2,3,6,7,8,10,11,12,13,14,15,17,18,19 %N A028733 Nonsquares mod 20. %t A028733 Complement[Range[0, 19], PowerMod[Range[20], 2, 20]] (* _Alonso del Arte_, Dec 20 2019 *) %o A028733 (Scala) (0 to 19).diff((1 to 20).map(n => (n * n) % 20)) // _Alonso del Arte_, Dec 20 2019 %Y A028733 Cf. A010382. %K A028733 nonn,fini,full,easy %O A028733 1,1 %A A028733 _N. J. A. Sloane_