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%I #9 Dec 27 2019 16:21:17
%S 2,3,5,6,8,11,12,14,15,17,18,20,21,23,24,26,27,29,30,32,33,35,38,39,
%T 41,42,44,45,47,48,50,51,53,54,56,57,59,60,62,65,66,68,69,71,72,74,75,
%U 77,78,80
%N Nonsquares mod 81.
%t Complement[Range[0, 80], PowerMod[Range[81], 2, 81]] (* _Alonso del Arte_, Dec 27 2019 *)
%o (Scala) (0 to 80).diff((1 to 81).map(n => (n * n) % 81)) // _Alonso del Arte_, Dec 27 2019
%Y Cf. A010442.
%K nonn,fini,full,easy
%O 1,1
%A _N. J. A. Sloane_