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%I #14 Dec 21 2019 00:52:44
%S 2,3,5,6,8,10,11,12,13,14,17,19,20
%N Nonsquares mod 21.
%e Since 14 is not a perfect square and there are no solutions to x^2 = 14 mod 21, 14 is in the sequence.
%e 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.
%t Complement[Range[20], PowerMod[Range[20], 2, 21]] (* _Alonso del Arte_, Nov 21 2019 *)
%o (Scala) val squaresMod21 = (0 to 20).map(n => n * n).map(_ % 21)
%o (0 to 20).diff(squaresMod21) // _Alonso del Arte_, Nov 21 2019
%Y Cf. A010383.
%K nonn,fini,full,easy
%O 1,1
%A _N. J. A. Sloane_
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