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%I #19 Dec 20 2019 20:45:45
%S 2,3,5,6,8,11,12,14,15,17
%N Nonsquares mod 18.
%e 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.
%e 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.
%t Complement[Range[17], PowerMod[Range[18], 2, 18]] (* _Alonso del Arte_, Nov 18 2019 *)
%o (Scala) val squaresMod18 = (0 to 17).map(n => n * n).map(_ % 18)
%o (0 to 17).diff(squaresMod18) // _Alonso del Arte_, Nov 18 2019
%o (PARI) is(n) = n<18 && !issquare(Mod(n, 18)) \\ _Felix Fröhlich_, Dec 18 2019
%Y Cf. A010380.
%Y Row 18 of A096013.
%K nonn,fini,full
%O 1,1
%A _N. J. A. Sloane_
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