A028794 -id:A028794 - cp's OEIS frontend

2, 3, 5, 6, 8, 11, 12, 14, 15, 17, 18, 20, 21, 23, 24, 26

Offset: 1

Cf. A010389. Subsequence of A028794.

Complement[Range[0, 26], PowerMod[Range[27], 2, 27]] (* Alonso del Arte, Nov 27 2019 *)

val squaresMod27 = (0 to 26).map(n => n * n).map(_ % 27)
(0 to 26).diff(squaresMod27) // Alonso del Arte, Nov 27 2019

0, 1, 4, 7, 9, 10, 13, 16, 19, 22, 25, 28, 31, 34, 36, 37, 40, 43, 46, 49, 52, 55, 58, 61, 63, 64, 67, 70, 73, 76, 79

Offset: 1

Union[PowerMod[Range[81], 2, 81]] (* Alonso del Arte, Dec 27 2019 *)

[quadratic_residues(81)] # Zerinvary Lajos, May 28 2009

(1 to 81).map(n => (n * n) % 81).toSet.toSeq.sorted // Alonso del Arte, Dec 27 2019

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