0
$\begingroup$

Let $K/\mathbb{Q}$ be an imaginary quadratic extension of discriminant $-D_K < -3$ and fix a prime $p > 3$ that is split in $K$ as $p\mathcal{O}_K = \mathfrak{p}\overline{\mathfrak{p}}$. Consider the character $$ \psi: I_K^{(p)} \rightarrow \{\pm 1\}$$ $$ \mathfrak{q} \mapsto \left(\frac{N(\mathfrak{q})}{p}\right)$$

where $N(\cdot)$ is the norm map, and $\left(\frac{\cdot}{p}\right)$ is the Legendre symbol. As far as I can see, $P_{K, 1}^{(p)} = \{x \in K \mid x - 1 \in p \mathcal{O}_K\}$ is in the kernel of this map. So this character defines a degree-$2$ extension of $K$, which we denote by $L$. The reason this is not the trivial character is because there exists a prime $q$ split in $K$ such that $(\frac{q}{p}) = -1$ (otherwise we would have $\mathbb{Q}(\sqrt{(-1)^{(p - 1)/2}p})$ be contained in $K$).

Here is an interesting property of $L$: $q$ is split in $L$ for every rational prime $q$ that is inert in $K$, because $\psi(q) = \left(\frac{N(q)}{p}\right) = \left(\frac{q^2}{p}\right) = 1$. So $L$ is contained in $H_K$, the Hilbert class field of $K$. But that's saying that the class number of $K$ must be even. Clearly, there are imaginary quadratic fields with odd class number. So, I would love to see where my mistake could have come from.

The reason I am interested in this character is, I was hoping that if I look at it as a Hecke character than the restriction to $\mathcal{O}_{K, \mathfrak{p}}^{\times}$ would also give me the (lift of) the Legendre symbol, which would be helpful for a problem that I am looking at.

$\endgroup$
3
  • 3
    $\begingroup$ I don't understand your proof that $L$ is contained in $H_K$. You need to prove that every principal prime ideal of $K$ splits in $L$, but you checked it only for inert primes (a trivial way of being principal). You extension smells ramified at $p$. $\endgroup$
    – Aurel
    Commented Nov 3 at 9:39
  • $\begingroup$ @Aurel thank you very much, you're right. Someone else also pointed out this mistake. I wasn't in my right mind to think that the inert primes are all of the principal ones. And indeed it has to be ramified somewhere, in which case it has to be ramified at $p$ if I'm not mistaken. $\endgroup$
    – Tam Nguyen
    Commented Nov 3 at 17:09
  • 2
    $\begingroup$ After a second look I realise your extension is actually $K(\sqrt{(-1)^{(p-1)/2}p})/K$, since $\mathop{Frob}_{\mathfrak{q}} \mapsto (N(\mathfrak{q}) \bmod p)$ is the mod $p$ cyclotomic character. In particular it is definitely ramified at the primes above $p$ under your hypotheses. $\endgroup$
    – Aurel
    Commented Nov 6 at 15:54

0

You must log in to answer this question.

Browse other questions tagged .