A006643 Class number of quadratic field with discriminant -4n as n runs through A089269: squarefree numbers congruent to 1 or 2 mod 4. (Formerly M0225)

1, 1, 2, 2, 2, 2, 4, 4, 4, 2, 6, 6, 4, 4, 4, 2, 6, 8, 4, 4, 6, 4, 2, 6, 8, 8, 8, 8, 4, 4, 10, 8, 4, 4, 4, 10, 12, 4, 8, 4, 14, 4, 8, 6, 6, 12, 8, 8, 6, 10, 12, 4, 4, 14, 8, 8, 8, 4, 8, 16, 14, 8, 6, 8, 16, 8, 10, 12, 14, 12, 4, 8, 10, 12, 16, 12, 4, 4, 20, 10, 12, 6, 8, 20, 20, 8, 8, 6, 8, 10, 16

Offset

1,3

Comments

Equivalently, number of classes of primitive positive definite binary quadratic forms of discriminant -4n as n runs through A089269: squarefree numbers congruent to 1 or 2 mod 4.

References

D. A. Buell, Binary Quadratic Forms. Springer-Verlag, NY, 1989, pp. 224-241.

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

Links

N. J. A. Sloane, Table of n, a(n) for n = 1..10000

Prog

(PARI) for (n=1, 50, if(issquarefree(n) && (n%4 == 1 || n%4 == 2), print(n, " ", qfbclassno(-4*n)))) \\ N. J. A. Sloane, May 28 2014

Crossrefs

Cf. A006642, A089269, A014599.

A subsequence of A000003.

Sequence in context: A331813 A215244 A195427 * A080217 A157901 A350697

Adjacent sequences: A006640 A006641 A006642 * A006644 A006645 A006646

Keyword

nonn

Author

N. J. A. Sloane, Mira Bernstein

Extensions

Extended and definition corrected by Max Alekseyev, Apr 16 2010

Status

approved