A028731 Nonsquares mod 18.

2, 3, 5, 6, 8, 11, 12, 14, 15, 17

Offset

1,1

Links

Table of n, a(n) for n=1..10.

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

Crossrefs

Cf. A010380.

Row 18 of A096013.

Sequence in context: A104424 A028806 A351807 * A028740 A028794 A028966

Adjacent sequences: A028728 A028729 A028730 * A028732 A028733 A028734

Keyword

nonn,fini,full

Author

N. J. A. Sloane

Status

approved