A028731 Nonsquares mod 18.
2, 3, 5, 6, 8, 11, 12, 14, 15, 17
Offset: 1
Examples
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.
Programs
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Mathematica
Complement[Range[17], PowerMod[Range[18], 2, 18]] (* Alonso del Arte, Nov 18 2019 *)
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PARI
is(n) = n<18 && !issquare(Mod(n, 18)) \\ Felix Fröhlich, Dec 18 2019
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Scala
val squaresMod18 = (0 to 17).map(n => n * n).map(_ % 18) (0 to 17).diff(squaresMod18) // Alonso del Arte, Nov 18 2019