Nonsquares mod 28.

Terms

a(0) =2a(1) =3a(2) =5a(3) =6a(4) =7a(5) =10a(6) =11a(7) =12a(8) =13a(9) =14a(10) =15a(11) =17a(12) =18a(13) =19a(14) =20a(15) =22a(16) =23a(17) =24a(18) =26a(19) =27