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Oct 6 at 8:09 comment added David Loeffler Yes, this is genus theory.
Oct 4 at 23:43 comment added Duality Is your claim regarding $Cl_K[2]$ in your answer a direct consequence of the so-called genus theory? If there are any formal documents (such as papers published in journals or books) that state the claim about $Cl_K[2]$, would it be possible for you to provide their references?
Jan 24 at 20:33 vote accept Duality
Aug 4, 2023 at 8:14 comment added David Loeffler Please don't ask new questions in comments.
Jul 18, 2023 at 9:20 comment added David Loeffler Yes, that's an obvious and well known corollary.
Jul 17, 2023 at 17:09 comment added Duality Thanks to your result, I thought that we can prove there are infinitely many quadratic fields with odd class number. There are infinitely many $K$ such that $Cl_K[2]=1$. Thus $Cl_K$ is odd abelian group!
Jul 16, 2023 at 15:14 comment added David Loeffler It's come up before on this site, see mathoverflow.net/questions/431499/…
Jul 16, 2023 at 14:13 comment added Duality Thank you very much. I searched a lot, but I couldn't find any reference for the fact that $\# Cl_K[2]=2^{r-1}$. Could you please provide me with a reference for that?
Jul 11, 2023 at 21:41 comment added David Loeffler @WillJagy I've added a clarification of exactly what I'm answering (since the title of the question, the first paragraph of the question, and the final paragraph are actually asking three distinct questions)
Jul 11, 2023 at 21:40 history edited David Loeffler CC BY-SA 4.0
added 247 characters in body
Jul 11, 2023 at 9:51 history answered David Loeffler CC BY-SA 4.0