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 A028731 #19 Dec 20 2019 20:45:45 %S A028731 2,3,5,6,8,11,12,14,15,17 %N A028731 Nonsquares mod 18. %e A028731 Since 6 is not a perfect square and there is no solution in integers to x^2 = 6 mod 18, 6 is in the sequence. %e A028731 Although 7 is not a perfect square either, we verify that x^2 = 7 mod 18 does have solutions, such as x = 5, x = 13. Therefore 7 is not in the sequence. %t A028731 Complement[Range[17], PowerMod[Range[18], 2, 18]] (* _Alonso del Arte_, Nov 18 2019 *) %o A028731 (Scala) val squaresMod18 = (0 to 17).map(n => n * n).map(_ % 18) %o A028731 (0 to 17).diff(squaresMod18) // _Alonso del Arte_, Nov 18 2019 %o A028731 (PARI) is(n) = n<18 && !issquare(Mod(n, 18)) \\ _Felix Fröhlich_, Dec 18 2019 %Y A028731 Cf. A010380. %Y A028731 Row 18 of A096013. %K A028731 nonn,fini,full %O A028731 1,1 %A A028731 _N. J. A. Sloane_