Example of a line bundle not admitting a $\operatorname{PGL}(n+1)$-linearization in Mumford's GIT $\DeclareMathOperator\PGL{PGL}\DeclareMathOperator\Proj{Proj}\DeclareMathOperator\Pic{Pic}$I have a question about an example for a line bundle not admitting a
$G$-linearization from Mumford's GIT, page 33:
We consider the action of $\PGL(n+1)$ on projecive space
$\mathbb{P}^n= \Proj k[X_0,\dotsc, X_n]$.
Observe that $\PGL(n+1)$ is given as the open affine
subscheme of
$$   \mathbb{P}^{n^2+2n} = \Proj k[a_{00},\dotsc,  a_{0n};
a_{10}, \dotsc   , a_{nn}]$$
complementary to the determinants hypersurface $\det(a_{ij})=0$. The
action morphism $  \sigma: \PGL(n+1) \times \mathbb{P}^n \to \mathbb{P}^n $ is determined by
\begin{gather*}
\sigma^*(\mathcal{O}_{\mathbb{P}^n}(1)) \cong
  p_1^*(\mathcal{O}_{\mathbb{P}^{n^2+2n}}(1)) \otimes 
p_2^*(\mathcal{O}_{\mathbb{P}^{n}}(1)) \\
\sigma^*(X_i)= \sum_{j=0}^n p_1^*(a_{ij}) \otimes p_2^*(X_j)
\end{gather*}
where $p_1, p_2$ are
projections canonical projections.
Mumford claims that $\mathcal{O}_{\mathbb{P}^n}(1) $ admits no
$\PGL(n+1)$-linearization, because
the restriction of $\mathcal{O}_{\mathbb{P}^{n^2+2n}}(1)$
to the open subscheme $\PGL(n+1)$ has order $n+1$ in
$\Pic[\PGL(n+1)]$, and is therefore not trivial.
My question is why the fact that $\mathcal{O}_{\mathbb{P}^{n^2+2n}}(1)$
restricted to the affine open $\PGL(n+1)$ is not trivial, implies that
$\mathcal{O}_{\mathbb{P}^n}(1) $ admits no
$\PGL(n+1)$-linearization?
[Indeed,  $\mathcal{O}_{\mathbb{P}^{n^2+2n}}(1)$ has order $n+1$ in
$V$ because $\Pic(\mathbb{P}^{n^2+2n}) \to
\Pic(\mathbb{P}^{n^2+2n}  \backslash V(\det(a_{ij}))=
\Pic(\PGL(n+1))$ induces an isomorphism
$\Pic(\mathbb{P}^{n^2+2n} \backslash V(\det(a_{ij})) \cong
\mathbb{Z}/(\deg(\det(a_{ij}))\mathbb{Z}$.]
To turn it another way round, why if $\mathcal{O}_{\mathbb{P}^n}(1) $
would admit a $\PGL(n+1)$-linearization, then
the restriction of $\mathcal{O}_{\mathbb{P}^{n^2+2n}}(1)$
to $\PGL(n+1)$ must be trivial? I conjecture that this argument can somehow reduced to an easy comparison of orders of group elements in groups $\Pic(X)$, $\Pic^G(X)$ but I do not see how it can be directly related.
Maybe it somehow helps to know that we have always a morphism of groups $   \Pic^G(X) \to \Pic(X)$ which is not neccessarily injective.
 A: $PGL(n+1)$-linearization of $\mathcal O_{\mathbb P^n}(1)$ could be used to produce isomorphism $\gamma : p_2^*(\mathcal O_{\mathbb P^n}(1)) \to \sigma^*(\mathcal O_{\mathbb P^n}(1))$, hence, taking to attention isomorphism
$$ \sigma^*(\mathcal{O}_{\mathbb{P}^n}(1)) \cong
  p_1^*(\mathcal{O}_{\mathbb{P}^{n^2+2n}}(1)) \otimes 
p_2^*(\mathcal{O}_{\mathbb{P}^{n}}(1))  $$
which you have mentioned, we have equality of Picard classes $([\mathcal{O}_{\mathbb{P}^{n^2+2n}}(1)],[\mathcal{O}_{\mathbb{P}^{n}}(1)])=(0,[\mathcal{O}_{\mathbb{P}^{n}}(1)])$ in $Pic(PGL(n+1)) 
\times Pic(\mathbb P^n) = Pic(PGL(n+1) \times \mathbb P^n)$, so, $[\mathcal{O}_{\mathbb{P}^{n^2+2n}}(1)] = 0 $ in $Pic(PGL(n+1))$
How to construct such $\gamma$? $PGL(n+1)$-linearization of $\mathcal O_{\mathbb P^n}(1)$ is a choice of morphism $\tilde{\sigma} : PGL(n+1) \times L \to L$  (where $L$ is a total space of the line bundle $\mathcal O_{\mathbb P^n}(1)$) such that the diagram
$$\require{AMScd}
\begin{CD}
p_2^*(\mathcal O_{\mathbb P^n}(1)) = PGL(n+1) \times L @>\tilde{\sigma}>> L\\
@V{}VV @V{}VV \\
PGL(n+1) \times \mathbb P^n @>{\sigma}>> \mathbb P^n
\end{CD}$$
commutes. Total space of the line bundle $\sigma^*(\mathcal O_{\mathbb P^n}(1))$ is by definition a fiber product $(PGL(n+1) \times \mathbb P^n) \times_{\mathbb P^n} L$, hence, by universal property of fiber products the morphism $\gamma : p_2^*(\mathcal O_{\mathbb P^n}(1)) \to \sigma^*(\mathcal O_{\mathbb P^n}(1))$ of $PGL(n+1) \times \mathbb P^n$-varieties exists. $\gamma$ is an isomorphism since for every $g \in PGL(n+1)$ induced morphism $\gamma_g : O_{\mathbb P^n}(1) \to g^* O_{\mathbb P^n}(1)$ is an isomorphism.
