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%I #17 Dec 21 2019 15:55:25
%S 0,1,4,7,9,15,16,18,21,22,25,28,30,36,37,39
%N Squares mod 42.
%t Union[PowerMod[Range[42], 2, 42]] (* _Alonso del Arte_, Dec 21 2019 *)
%o (Sage) [quadratic_residues(42)] # _Zerinvary Lajos_, May 24 2009
%o (Scala) (1 to 42).map(n => (n * n) % 42).toSet.toSeq.sorted // _Alonso del Arte_, Dec 21 2019
%Y Cf. A028755.
%K nonn,fini,full,easy
%O 1,3
%A _N. J. A. Sloane_