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 A028731 Nonsquares mod 18. 2
 2, 3, 5, 6, 8, 11, 12, 14, 15, 17 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS EXAMPLE Since 6 is not a perfect square and there is no solution in integers to x^2 = 6 mod 18, 6 is in the sequence. Although 7 is not a perfect square either, we verify that x^2 = 7 mod 18 does have solutions, such as x = 5, x = 13. Therefore 7 is not in the sequence. MATHEMATICA Complement[Range[17], PowerMod[Range[18], 2, 18]] (* Alonso del Arte, Nov 18 2019 *) PROG (Scala) val squaresMod18 = (0 to 17).map(n => n * n).map(_ % 18) (0 to 17).diff(squaresMod18) // Alonso del Arte, Nov 18 2019 (PARI) is(n) = n<18 && !issquare(Mod(n, 18)) \\ Felix FrÃ¶hlich, Dec 18 2019 CROSSREFS Cf. A010380. Row 18 of A096013. Sequence in context: A188064 A104424 A028806 * A028740 A028794 A028966 Adjacent sequences:  A028728 A028729 A028730 * A028732 A028733 A028734 KEYWORD nonn,fini,full AUTHOR STATUS approved

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Last modified September 19 00:22 EDT 2021. Contains 347549 sequences. (Running on oeis4.)