Skip to main content
added 451 characters in body
Source Link
Francesco Polizzi
  • 66.3k
  • 5
  • 180
  • 283

Yes, rationally connected varieties in characteristic zero are uniruled (a fact which follows essentially by definition).

OneIn characteristic zero, one can think about this also by exploiting the following characterization of uniruled and rationally connected varieties, that can be found in Debarre's book Higher dimensional algebraic geometry, Chapter 4.

Let $X$ be a smooth projective variety of dimension $d$, defined over a field $\mathbb{K}$ of characteristic zero. Recall that a rational curve $f \colon \mathbb{P}^1 \to X$ is called $r$-free if $f^*T_X \otimes \mathscr{O}_{\mathbb{P}^1}(-r)$ is generated by its global sections. By Grothendieck' s Theorem any vector bundle over $\mathbb{P}^1$ splits as a direct sum of line bundles, so $r$-free means $$f^*T_X = \mathscr{O}_{\mathbb{P}^1}(a_1) \oplus \ldots \oplus \mathscr{O}_{\mathbb{P}^1}(a_d),$$ with $a_i \geq r$ for every $i$. Now one has the following

Proposition. Assume $\textrm{char}(\mathbb{K})=0$. Then $X$ is uniruled if and only through a general point $x \in X$ there is a $0$-free rational curve. Moreover $X$ is rationally connected if and only if through a general point $x \in X$ there is a $1$-free rational curve.

Remark. The condition $\textrm{char}(\mathbb{K})=0$ is an essential one. For instance, if $\textrm{char}(\mathbb{K})=p>0$ then the Fermat hypersurface of degree $p^r+1$ in $\mathbb{P}^N$, with $N \geq 4$ and $r \geq 1$, is uniruled by lines, none of which are free. Moreover, if $p^r >N$ any such a hypersurface has ample canonical class, then it contains no free rational curves at all.

Yes, rationally connected varieties in characteristic zero are uniruled (a fact which follows essentially by definition).

One can think about this also by exploiting the following characterization of uniruled and rationally connected varieties, that can be found in Debarre's book Higher dimensional algebraic geometry, Chapter 4.

Let $X$ be a smooth projective variety of dimension $d$, defined over a field of characteristic zero. Recall that a rational curve $f \colon \mathbb{P}^1 \to X$ is called $r$-free if $f^*T_X \otimes \mathscr{O}_{\mathbb{P}^1}(-r)$ is generated by its global sections. By Grothendieck' s Theorem any vector bundle over $\mathbb{P}^1$ splits as a direct sum of line bundles, so $r$-free means $$f^*T_X = \mathscr{O}_{\mathbb{P}^1}(a_1) \oplus \ldots \oplus \mathscr{O}_{\mathbb{P}^1}(a_d),$$ with $a_i \geq r$ for every $i$. Now one has the following

Proposition. $X$ is uniruled if and only through a general point $x \in X$ there is a $0$-free rational curve. Moreover $X$ is rationally connected if and only if through a general point $x \in X$ there is a $1$-free rational curve.

Yes, rationally connected varieties are uniruled (a fact which follows essentially by definition).

In characteristic zero, one can think about this also by exploiting the following characterization of uniruled and rationally connected varieties, that can be found in Debarre's book Higher dimensional algebraic geometry, Chapter 4.

Let $X$ be a smooth projective variety of dimension $d$, defined over a field $\mathbb{K}$ of characteristic zero. Recall that a rational curve $f \colon \mathbb{P}^1 \to X$ is called $r$-free if $f^*T_X \otimes \mathscr{O}_{\mathbb{P}^1}(-r)$ is generated by its global sections. By Grothendieck' s Theorem any vector bundle over $\mathbb{P}^1$ splits as a direct sum of line bundles, so $r$-free means $$f^*T_X = \mathscr{O}_{\mathbb{P}^1}(a_1) \oplus \ldots \oplus \mathscr{O}_{\mathbb{P}^1}(a_d),$$ with $a_i \geq r$ for every $i$. Now one has the following

Proposition. Assume $\textrm{char}(\mathbb{K})=0$. Then $X$ is uniruled if and only through a general point $x \in X$ there is a $0$-free rational curve. Moreover $X$ is rationally connected if and only if through a general point $x \in X$ there is a $1$-free rational curve.

Remark. The condition $\textrm{char}(\mathbb{K})=0$ is an essential one. For instance, if $\textrm{char}(\mathbb{K})=p>0$ then the Fermat hypersurface of degree $p^r+1$ in $\mathbb{P}^N$, with $N \geq 4$ and $r \geq 1$, is uniruled by lines, none of which are free. Moreover, if $p^r >N$ any such a hypersurface has ample canonical class, then it contains no free rational curves at all.

added 48 characters in body
Source Link
Francesco Polizzi
  • 66.3k
  • 5
  • 180
  • 283

The answer is yesYes and it is a well known standard fact, rationally connected varieties in Algebraic Geometrycharacteristic zero are uniruled (a fact which follows essentially by definition).

One can think about this topic also by looking atexploiting the following characterization of uniruled and rationally connected varieties, that can be found in Debarre's book Higher dimensional algebraic geometry, Chapter 4.

Let $X$ be a smooth projective variety of dimension $d$, defined over a field of characteristic zero. Recall that a rational curve $f \colon \mathbb{P}^1 \to X$ is called $r$-free if $f^*T_X \otimes \mathscr{O}_{\mathbb{P}^1}(-r)$ is generated by its global sections. By Grothendieck' s Theorem any vector bundle over $\mathbb{P}^1$ splits as a direct sum of line bundles, so $r$-free means $$f^*T_X = \mathscr{O}_{\mathbb{P}^1}(a_1) \oplus \ldots \oplus \mathscr{O}_{\mathbb{P}^1}(a_d),$$ with $a_i \geq r$ for every $i$. Now one has the following

Proposition. $X$ is uniruled if and only through anya general point $x \in X$ there is a $0$-free rational curve. Moreover $X$ is rationally connected if and only if through anya general point $x \in X$ there is a $1$-free rational curve.

The answer is yes and it is a well known standard fact in Algebraic Geometry.

One can think about this topic also by looking at the following characterization of uniruled and rationally connected varieties, that can be found in Debarre's book Higher dimensional algebraic geometry, Chapter 4.

Let $X$ be a smooth projective variety of dimension $d$, defined over a field of characteristic zero. Recall that a rational curve $f \colon \mathbb{P}^1 \to X$ is called $r$-free if $f^*T_X \otimes \mathscr{O}_{\mathbb{P}^1}(-r)$ is generated by its global sections. By Grothendieck' s Theorem any vector bundle over $\mathbb{P}^1$ splits as a direct sum of line bundles, so $r$-free means $$f^*T_X = \mathscr{O}_{\mathbb{P}^1}(a_1) \oplus \ldots \oplus \mathscr{O}_{\mathbb{P}^1}(a_d),$$ with $a_i \geq r$ for every $i$. Now one has the following

Proposition. $X$ is uniruled if and only through any point $x \in X$ there is a $0$-free rational curve. Moreover $X$ is rationally connected if and only if through any point $x \in X$ there is a $1$-free rational curve.

Yes, rationally connected varieties in characteristic zero are uniruled (a fact which follows essentially by definition).

One can think about this also by exploiting the following characterization of uniruled and rationally connected varieties, that can be found in Debarre's book Higher dimensional algebraic geometry, Chapter 4.

Let $X$ be a smooth projective variety of dimension $d$, defined over a field of characteristic zero. Recall that a rational curve $f \colon \mathbb{P}^1 \to X$ is called $r$-free if $f^*T_X \otimes \mathscr{O}_{\mathbb{P}^1}(-r)$ is generated by its global sections. By Grothendieck' s Theorem any vector bundle over $\mathbb{P}^1$ splits as a direct sum of line bundles, so $r$-free means $$f^*T_X = \mathscr{O}_{\mathbb{P}^1}(a_1) \oplus \ldots \oplus \mathscr{O}_{\mathbb{P}^1}(a_d),$$ with $a_i \geq r$ for every $i$. Now one has the following

Proposition. $X$ is uniruled if and only through a general point $x \in X$ there is a $0$-free rational curve. Moreover $X$ is rationally connected if and only if through a general point $x \in X$ there is a $1$-free rational curve.

Source Link
Francesco Polizzi
  • 66.3k
  • 5
  • 180
  • 283

The answer is yes and it is a well known standard fact in Algebraic Geometry.

One can think about this topic also by looking at the following characterization of uniruled and rationally connected varieties, that can be found in Debarre's book Higher dimensional algebraic geometry, Chapter 4.

Let $X$ be a smooth projective variety of dimension $d$, defined over a field of characteristic zero. Recall that a rational curve $f \colon \mathbb{P}^1 \to X$ is called $r$-free if $f^*T_X \otimes \mathscr{O}_{\mathbb{P}^1}(-r)$ is generated by its global sections. By Grothendieck' s Theorem any vector bundle over $\mathbb{P}^1$ splits as a direct sum of line bundles, so $r$-free means $$f^*T_X = \mathscr{O}_{\mathbb{P}^1}(a_1) \oplus \ldots \oplus \mathscr{O}_{\mathbb{P}^1}(a_d),$$ with $a_i \geq r$ for every $i$. Now one has the following

Proposition. $X$ is uniruled if and only through any point $x \in X$ there is a $0$-free rational curve. Moreover $X$ is rationally connected if and only if through any point $x \in X$ there is a $1$-free rational curve.