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 A028732 #14 Dec 19 2019 23:19:05 %S A028732 2,3,8,10,12,13,14,15,18 %N A028732 Nonsquares mod 19. %e A028732 Since 10 is not a perfect square, and there are no solutions in integers to x^2 = 10 mod 19, 10 is in the sequence. %e A028732 Although 11 is not a perfect square either, there are solutions in integers to x^2 = 11 mod 19, such as x = 7, x = 12. Hence 11 is not in the sequence. %t A028732 Select[Range[18], JacobiSymbol[#, 19] != 1 &] (* _Alonso del Arte_, Nov 19 2019 *) %o A028732 (Scala) val squaresMod19 = (0 to 18).map(n => n * n).map(_ % 19) %o A028732 (0 to 18).diff(squaresMod19) // _Alonso del Arte_, Nov 19 2019 %K A028732 nonn,fini,full %O A028732 1,1 %A A028732 _N. J. A. Sloane_