OFFSET
1,1
EXAMPLE
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.
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.
MATHEMATICA
Complement[Range[17], PowerMod[Range[18], 2, 18]] (* Alonso del Arte, Nov 18 2019 *)
PROG
(Scala) val squaresMod18 = (0 to 17).map(n => n * n).map(_ % 18)
(0 to 17).diff(squaresMod18) // Alonso del Arte, Nov 18 2019
(PARI) is(n) = n<18 && !issquare(Mod(n, 18)) \\ Felix Fröhlich, Dec 18 2019
KEYWORD
nonn,fini,full
AUTHOR
STATUS
approved