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 A028773 #12 Jan 04 2020 02:56:23 %S A028773 2,3,5,6,7,8,10,11,12,13,14,15,17,18,19,20,22,23,26,27,28,29,30,31,32, %T A028773 33,34,35,37,38,39,41,42,43,44,46,47,48,50,51,52,53,54,55,56,57,58,59 %N A028773 Nonsquares mod 60. %e A028773 x^2 = 20 mod 60 has no solutions, so 20 is in the sequence. %e A028773 x^2 = 21 mod 60 has the following solutions: x = 9, x = 21, x = 39, x = 51. Hence 21 is not in the sequence. %t A028773 Complement[Range[59], PowerMod[Range[60], 2, 60]] (* _Alonso del Arte_, Jan 13 2017 *) %o A028773 (Scala) (0 to 59).diff((1 to 60).map(n => n * n % 60)) // _Alonso del Arte_, Jan 03 2020 %Y A028773 Cf. A010421. %K A028773 nonn,fini,full,easy %O A028773 1,1 %A A028773 _N. J. A. Sloane_