Skip to main content
5 of 5
Fixed proof to include the top exterior power.
Jason Starr
  • 4.1k
  • 1
  • 93
  • 111

The projective space associated to a finite-dimensional vector space $V$ over a field $k$ is a universal pair $(\mathbb{P}V,\tilde{\gamma})$ of a $k$-scheme $\mathbb{P}V$ and a surjection of coherent sheaves $$ \tilde{\gamma}:V^\vee \otimes_k \mathcal{O}_{\mathbb{P}V} \to \mathcal{O}_{\mathbb{P}V}(1), $$ such that $\mathcal{O}_{\mathbb{P}V}(1)$ is an invertible sheaf (the Serre twisting sheaf). $\textbf{NB}.$ Some people prefer to use $V$ rather than $V^\vee$ in this definition.

Tensoring $\tilde{\gamma}$ by the identity on $\mathcal{O}_{\mathbb{P}V}(-1)$ gives another morphism of coherent sheaves,

$$ \gamma:V^\vee\otimes_k \mathcal{O}_{\mathbb{P}V} (-1) \to \mathcal{O}_{\mathbb{P}V}.$$

As a map to the structure sheaf, we can use this to form a Koszul complex $(K_\bullet,d_\bullet)$ where the term $K_p$ is

$$\bigwedge_{\mathcal{O}}^p (V^\vee\otimes_k \mathcal{O}_{\mathbb{P}V}(-1) ) \cong (\bigwedge_k^p V^\vee)\otimes_k \mathcal{O}_{\mathbb{P}V}(-p),$$

and where the differentials $d_\bullet$ are the unique morphisms of coherent sheaves such that $d_1$ equals $\gamma$, and such that $(K_\bullet,d_\bullet)$ is a differential graded algebra, i.e., the differentials satisfy the Leibniz rule for exterior product.

Because $\gamma$ is surjective, this complex is exact. In particular, if we define $S_p$ to be the $p^\text{th}$ syzygy, i.e., the kernel of $d_p:K_p \to K_{p-1}$, then we can break up the complex into a sequence of short exact sequences,

$$ 0 \to S_{p+1} \to K_{p+1} \to S_p \to 0. $$

Moreover, because $(K_\bullet,d_\bullet)$ is a differential graded algebra, there are cup product maps

$$ \bigwedge^p S_1 \to S_p, $$

which turn out to be isomorphisms (easiest to check locally, where $\gamma$ splits). Finally, the Euler sequence identifies $S_1$ as $\Omega_{\mathbb{P}V/k}$. Therefore the short exact sequences above give

$$ 0 \to \Omega^{p+1}_{\mathbb{P}V/k} \to (\bigwedge^{p+1}_k V^\vee)\otimes_k \mathcal{O}_{\mathbb{P}V}(-p-1) \to \Omega^p_{\mathbb{P}V} \to 0. $$

From this it follows immediately that $\Omega^p_{\mathbb{P}V}(p+1)$ is globally generated for every $p\geq 1$.

EDIT. The argument above is only valid for $1\leq p \leq n-1$. But the short exact sequence also proves that $\Omega^n_{\mathbb{P}V} \cong (\bigwedge^{n+1}_k V^\vee) \otimes_k \mathcal{O}_{\mathbb{P}V}(-n-1)$. So when $p$ equals $n$, also $\Omega^n_{\mathbb{P}V}(n+1) \cong (\bigwedge^{n+1}_k V^\vee)\otimes_k \mathcal{O}_{\mathbb{P}V}$, which is also globally generated.

Jason Starr
  • 4.1k
  • 1
  • 93
  • 111