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%I #9 Dec 13 2019 14:03:15
%S 2,5,6,7,8,11
%N Nonsquares mod 13.
%e Since 2 is not a perfect square and there are no solutions to x^2 = 2 mod 13, 2 is in the sequence.
%e Although 3 is not a perfect square either, there are solutions to x^2 = 3 mod 13, such as x = 4, x = 9. Hence 3 is not in the sequence.
%t Select[Range[0, 12], JacobiSymbol[#, 13] == -1 &] (* _Alonso del Arte_, Dec 13 2019 *)
%o (Scala) (0 to 12).diff((1 to 13).map(n => (n * n) % 13)) // _Alonso del Arte_, Dec 13 2019
%Y Cf. A010376.
%K nonn,fini,full
%O 1,1
%A _N. J. A. Sloane_
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