This site is supported by donations to The OEIS Foundation.

 Please make a donation to keep the OEIS running. We are now in our 55th year. In the past year we added 12000 new sequences and reached 8000 citations (which often say "discovered thanks to the OEIS"). We need to raise money to hire someone to manage submissions, which would reduce the load on our editors and speed up editing. Other ways to donate

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 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 (list; graph; refs; listen; history; text; internal format)
 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: A158502 A215244 A195427 * A080217 A157901 A327441 Adjacent sequences:  A006640 A006641 A006642 * A006644 A006645 A006646 KEYWORD nonn AUTHOR EXTENSIONS Extended and definition corrected by Max Alekseyev, Apr 16 2010 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified December 12 15:11 EST 2019. Contains 329960 sequences. (Running on oeis4.)