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Squares mod 28.
1

%I #18 Nov 28 2019 14:46:40

%S 0,1,4,8,9,16,21,25

%N Squares mod 28.

%t Union[PowerMod[Range[28], 2, 28]] (* _Alonso del Arte_, Nov 28 2019 *)

%o (Sage) [quadratic_residues(28)] # _Zerinvary Lajos_, May 24 2009

%o (Scala) (1 to 28).map(n => n * n).map(_ % 28).toSet.toSeq.sorted // _Alonso del Arte_, Nov 28 2019

%Y Cf. A028741.

%K nonn,fini,full

%O 1,3

%A _N. J. A. Sloane_