A028734 Nonsquares mod 21.

2, 3, 5, 6, 8, 10, 11, 12, 13, 14, 17, 19, 20

Offset

1,1

Links

Table of n, a(n) for n=1..13.

Example

Since 14 is not a perfect square and there are no solutions to x^2 = 14 mod 21, 14 is in the sequence.
Although 15 is not a perfect square either, there are solutions to x^2 = 15 mod 21, such as x = 6, x = 15. Hence 15 is not in the sequence.

Mathematica

Complement[Range[20], PowerMod[Range[20], 2, 21]] (* Alonso del Arte, Nov 21 2019 *)

Prog

(Scala) val squaresMod21 = (0 to 20).map(n => n * n).map(_ % 21)
(0 to 20).diff(squaresMod21) // Alonso del Arte, Nov 21 2019

Crossrefs

Cf. A010383.

Sequence in context: A050578 A283513 A028776 * A028755 A187897 A187319

Adjacent sequences: A028731 A028732 A028733 * A028735 A028736 A028737

Keyword

nonn,fini,full,easy

Author

N. J. A. Sloane

Status

approved