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 A039957 Squarefree numbers congruent to 3 mod 4. 12
 3, 7, 11, 15, 19, 23, 31, 35, 39, 43, 47, 51, 55, 59, 67, 71, 79, 83, 87, 91, 95, 103, 107, 111, 115, 119, 123, 127, 131, 139, 143, 151, 155, 159, 163, 167, 179, 183, 187, 191, 195, 199, 203, 211, 215, 219, 223, 227, 231, 235, 239, 247, 251, 255 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Negatives of odd fundamental discriminants D := b^2-4*a*c<0 of definite integer binary quadratic forms F=a*x^2+b*x*y+c*y^2. See Buell reference pp. 224-230. See 4*A089269 = A191483 for the even case and A003657 for combined even and odd numbers. - Wolfdieter Lang, Nov 07 2003 The asymptotic density of this sequence is 2/Pi^2 = 0.202642... (A185197). - Amiram Eldar, Feb 10 2021 REFERENCES Richard A. Mollin, Quadratics, CRC Press, 1996, Tables B1-B3. Duncan A. Buell, Binary Quadratic Forms, Springer-Verlag, NY, 1989. LINKS Reinhard Zumkeller, Table of n, a(n) for n = 1..10000 A. M. Legendre, Diviseurs de la forme t^2+au^2 a étant un nombre de la forme 4n-1, Essai sur la Théorie des Nombres An VI, Table V. MATHEMATICA fQ[n_] := SquareFreeQ[n] && Mod[n, 4] == 3; Select[ Range@ 258, fQ] (* Robert G. Wilson v, Mar 02 2011 *) Select[Range[3, 300, 4], SquareFreeQ] (* Harvey P. Dale, Mar 08 2015 *) PROG (MAGMA) [4*n+3: n in [0..63] | IsSquarefree(4*n+3)];  // Bruno Berselli, Mar 04 2011 (Haskell) a039957 n = a039957_list !! (n-1) a039957_list = filter ((== 3) . (`mod` 4)) a005117_list -- Reinhard Zumkeller, Aug 15 2011 (PARI) is(n)=n%4==3 && issquarefree(n) \\ Charles R Greathouse IV, Feb 07 2017 CROSSREFS Cf. A039955, A039956, A185197, A191483. Sequence in context: A118894 A194397 A330213 * A217332 A079422 A310210 Adjacent sequences:  A039954 A039955 A039956 * A039958 A039959 A039960 KEYWORD nonn,easy,nice AUTHOR EXTENSIONS Offset corrected STATUS approved

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Last modified April 17 14:32 EDT 2021. Contains 343063 sequences. (Running on oeis4.)