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I would like to prove $ \Bbb Q_p^×/(\Bbb Q_p^×)^p$ is isomorphic to $\Bbb Z/p\Bbb Z×\Bbb Z/p\Bbb Z$ as a group.

I know $\Bbb Q_p^×$ is isomorphic to $\Bbb Z×\Bbb Z_p×\Bbb Z/(p-1)\Bbb Z$ using formal logarithm.

I tried to find group homomorphism $\Bbb Q_p^×$$\Bbb Z/p\Bbb Z×\Bbb Z/p\Bbb Z$ of which kenerl is $(\Bbb Q_p^×)^p$, but I couldn't.

Thank you in advance.

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  • $\begingroup$ Thank you!, but is $G^p$ isomorphic to $\Bbb pZ×\Bbb pZ_p×\Bbb Z/(p-1)\Bbb Z$? $\endgroup$ Commented Mar 12, 2021 at 13:27
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    $\begingroup$ This is false when $p = 2$: the group $\mathbf Q_2^\times/(\mathbf Q_2^\times)^2$ has order $8$, not $4$. $\endgroup$
    – KCd
    Commented Mar 12, 2021 at 20:07

1 Answer 1

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Is it clear to you that (Hensel lemma) for $p$ odd: $$\Bbb{Q}_p^\times = p^\Bbb{Z}\times (1+p\Bbb{Z}_p)\times \langle \zeta_{p-1}\rangle$$

Then $(1+p\Bbb{Z}_p)^p=1+p^2\Bbb{Z}_p$ because it contains $(1+cp^k)^p\equiv 1+ c p^{k+1}\bmod p^{k+2}$ so any $1+p^2 a$ is of the form $\prod_{k\ge 1} (1+c_k p^k)^p$ with $c_k\in 0 \ldots p-1$.

Whence $$\Bbb{Q}_p^\times/(\Bbb{Q}_p^\times)^p\cong p^\Bbb{Z}/p^\Bbb{pZ}\times (1+p\Bbb{Z}_p)/(1+p^2\Bbb{Z}_p)\cong \Bbb{F}_p\times \Bbb{F}_p$$

For $p=2$ it is the same except that $(1+2\Bbb{Z}_2)^2=1+8\Bbb{Z}_2$ so $\Bbb{Q}_2^\times/(\Bbb{Q}_2^\times)^2\cong \Bbb{F}_2\times \Bbb{F}_2\times \Bbb{F}_2$.

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