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%I #19 Dec 21 2019 18:23:34
%S 0,1,4,7,9,15,16,18
%N Squares mod 21.
%t Union[PowerMod[Range[21], 2, 21]] (* _Alonso del Arte_, Dec 21 2019 *)
%o (Sage) [quadratic_residues(21)] # _Zerinvary Lajos_, May 24 2009
%o (Scala) (1 to 21).map(n => (n * n) % 21).toSet.toSeq.sorted // _Alonso del Arte_, Dec 21 2019
%Y Row 21 of A096008.
%Y Cf. A028734.
%K nonn,fini,full,easy
%O 1,3
%A _N. J. A. Sloane_
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