

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)


3



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
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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. SpringerVerlag, NY, 1989, pp. 224241.
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: A158502 A215244 A195427 * A080217 A157901 A327441
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



