The Stacks project

Proof. Let $g_1, \ldots , g_ r \in G$ be elements which topologically generate $G$, i.e., this means that the subgroup generated by $g_1, \ldots , g_ r$ is dense. By Lemma 59.57.4 we see that $H^1(G, V)$ is the $k$-vector space of extensions

of $k\text{-}G$-modules. Choose $e \in E$ mapping to $1 \in k$. Write

for some $v_ i \in V$. This is possible because $g_ i \cdot 1 = 1$. We claim that the list of elements $v_1, \ldots , v_ r \in V$ determine the isomorphism class of the extension $E$. Once we prove this the lemma follows as this means that our Ext vector space is isomorphic to a subquotient of the $k$-vector space $V^{\oplus r}$; some details omitted. Since $E$ is an object of the category defined in Definition 59.57.1 we know there is an open subgroup $U$ such that $u \cdot e = e$ for all $u \in U$. Now pick any $g \in G$. Then $gU$ contains a word $w$ in the elements $g_1, \ldots , g_ r$. Say $gu = w$. Since the element $w \cdot e$ is determined by $v_1, \ldots , v_ r$, we see that $g \cdot e = (gu) \cdot e = w \cdot e$ is too. $\square$