

A028731


Nonsquares mod 18.


2




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 FrÃ¶hlich, Dec 18 2019


CROSSREFS

Cf. A010380.
Row 18 of A096013.
Sequence in context: A188064 A104424 A028806 * A028740 A028794 A028966
Adjacent sequences: A028728 A028729 A028730 * A028732 A028733 A028734


KEYWORD

nonn,fini,full


AUTHOR

N. J. A. Sloane


STATUS

approved



