1
$\begingroup$

Let $p$ be a prime and denote by $\mathbb{Z}_p^{\times}$ the group of $p$-adic units. Suppose that $\chi$ is a character $\chi: \mathbb{Z}_p^{\times} \rightarrow \mathbb{C}^{\times}$. Then it is well known that

$\mathbb{Z}_p^{\times} \simeq (\mathbb{Z}/ p \mathbb{Z})^{\times} \times (1+ p \mathbb{Z}_p) $.

Do we have that the kernel of $\chi$ contains $(1+ p \mathbb{Z}_p)$ ? Any help or reference would be appreciated.

$\endgroup$
1
  • 2
    $\begingroup$ No, Pontryagin duality ensures continuous homomorphisms to the unit circle separate points. And actually it's not needed: just consider for $n\ge 2$ the reduction map $Z_p^*\to (Z/p^nZ)^*$. $\endgroup$
    – YCor
    Aug 11, 2021 at 7:02

1 Answer 1

2
$\begingroup$

YCor's example in the comments shows that you can have continuous characters with image an arbitrarily large finite group. In fact, there are even injective homomorphisms $\mathbb{Z}_p^\times \to \mathbb{C}^\times$ (if you don't ask for continuity): Observe that $\mathbb{C}^\times \cong \mathbb{Q}/\mathbb{Z}\times \bigoplus \mathbb{Q}$ as abstract abelian group, where $\bigoplus \mathbb{Q}$ is a $\mathbb{Q}$-vector space of continuum dimension.

For odd $p$, we have $\mathbb{Z}_p^\times \cong \mathbb{F}_p^\times \times \mathbb{Z}_p$. You can map the cyclic group $\mathbb{F}_p^\times$ injectively into the $\mathbb{Q}/\mathbb{Z}$ factor, and $\mathbb{Z}_p$ injectively into the continuum-dimensional $\mathbb{Q}$-vector space, since $\mathbb{Z}_p$ is torsion free and $\mathbb{Z}_p\otimes \mathbb{Q}$ is also a continuum-dimensional $\mathbb{Q}$-vector space.

For $p=2$, we abstractly have $\mathbb{Z}_2^\times \cong \{\pm 1\} \times \mathbb{Z}_2$, so the same argument works.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.