# divisibility of class numbers of composite two abelian number fields

Let $p$ be a regular prime i.e. $p\nmid h_p$, ($h_p$ denotes the class number of $\mathbb{Q}(\mu_p)$). Let $F$ be an abelian number field such that $p\nmid h_F$ and $p\nmid [F:\mathbb{Q}]$.

Do we have $p\nmid h_{F(\mu_p)}$?

I hope it holds, but in general the following is wrong:

Let $K,L$ be two number fields such that $p\nmid h_K h_L$, and $p\nmid [KL:K][KL:L]$, then $p\nmid h_{KL}$.

It's easy to find lots of examples from quadratic fields, for instance, take $p=3, K=\mathbb{Q}(i),L=\mathbb{Q}(\sqrt{23})$. $h_{KL}=3$

• Download pari, familiarize yourself with the program, and find as many counter-examples as you like. – franz lemmermeyer Sep 7 '17 at 9:10
• @franzlemmermeyer Thank you a lot! I'll delete this question. – J.Li Sep 21 '17 at 1:28