First Characterization of Ample Divisors

Oct 13, 2025 by wbrum.org 105 views

Introduction

Toric geometry is a branch of algebraic geometry that studies algebraic varieties that are invariant under the action of a torus. In Fulton's book "Toric Varieties", he provides a comprehensive treatment of toric geometry, including the definition of strictly convex functions on a fan. In this article, we will discuss two characterizations of ample divisors in toric geometry, as defined by Fulton.

Background

A fan $\Delta$ in a lattice $N$ is a finite collection of cones in $N \otimes \mathbb{R}$ such that for any cone $\sigma \in \Delta$ , there exists a finite collection of cones $\sigma_i \in \Delta$ such that $\sigma = \bigcup \sigma_i$ . A convex PL function $\psi$ on a fan $\Delta$ is a function that is convex on each cone of $\Delta$ and is a piecewise linear function on the cones of $\Delta$ . Fulton defines a strictly convex function on a fan $\Delta$ in two ways.

First Characterization

The first characterization of a strictly convex function on a fan $\Delta$ is as follows:

A convex PL function $\psi$ on a fan $\Delta$ is strictly convex if the graph of $\psi$ on the cone $\sigma \in \Delta$ is a strictly convex set.

To understand this definition, let's consider the graph of $\psi$ on the cone $\sigma \in \Delta$ . The graph of $\psi$ on $\sigma$ is the set of points $(x, \psi(x))$ where $x \in \sigma$ . A set is strictly convex if it does not contain any line segment. In other words, if $x, y \in \sigma$ and $x \neq y$ , then the line segment joining $x$ and $y$ does not lie in the graph of $\psi$ on $\sigma$ .

Second Characterization

The second characterization of a strictly convex function on a fan $\Delta$ is as follows:

A convex PL function $\psi$ on a fan $\Delta$ is strictly convex if for any two points $x, y \in \sigma \in \Delta$ , the line segment joining $x$ and $y$ does not lie in the graph of $\psi$ on $\sigma$ .

This definition is equivalent to the first characterization. To see this, let's consider the graph of $\psi$ on the cone $\sigma \in \Delta$ . If the graph of $\psi$ on $\sigma$ is a strictly convex set, then for any two points $x, y \in \sigma$ , the line segment joining $x$ and $y$ does not lie in the graph of $\psi$ on $\sigma$ . Conversely, if for any two points $x, y \in \sigma$ , the line segment joining $x$ and $y$ does not lie in the graph of $\psi$ on $\sigma$ , then the graph of $\psi$ on $\sigma$ is a strictly convex set.

Ample Divisors

An ample divisor on a toric variety is a divisor that is ample in the sense of algebraic geometry. In other words, an ample divisor on a toric variety is a divisor that is very ample and has a positive self-intersection number.

First Characterization of Ample Divisors

The first characterization of an ample divisor on a toric variety is as follows:

A divisor $D$ on a toric variety $X$ is ample if and only if the function $\psi_D$ associated with $D$ is strictly convex on the fan $\Delta$ of $X$ .

Here, $\psi_D$ is the function associated with the divisor $D$ and is defined as follows:

For any point $x \in X$ , $\psi_D(x) = \sum_{i=1}^n a_i x_i$ where $a_i$ are the coefficients of the divisor $D$ and $x_i$ are the coordinates of the point $x$ .

Second Characterization of Ample Divisors

The second characterization of an ample divisor on a toric variety is as follows:

A divisor $D$ on a toric variety $X$ is ample if and only if for any two points $x, y \in X$ , the line segment joining $x$ and $y$ does not lie in the graph of $\psi_D$ on the cone $\sigma \in \Delta$ associated with $D$ .

This characterization is equivalent to the first characterization. To see this, let's consider the function $\psi_D$ associated with the divisor $D$ . If the function $\psi_D$ is strictly convex on the fan $\Delta$ of $X$ , then for any two points $x, y \in X$ , the line segment joining $x$ and $y$ does not lie in the graph of $\psi_D$ on the cone $\sigma \in \Delta$ associated with $D$ . Conversely, if for any two points $x, y \in X$ , the line segment joining $x$ and $y$ does not lie in the graph of $\psi_D$ on the cone $\sigma \in \Delta$ associated with $D$ , then the function $\psi_D$ is strictly convex on the fan $\Delta$ of $X$ .

Conclusion

In this article, we have discussed two characterizations of ample divisors in toric geometry, as defined by Fulton. The first characterization is based on the definition of a strictly convex function on a fan, while the second characterization is based on the property of the graph of the function associated with the divisor. We have shown that these two characterizations are equivalent and provide a complete characterization of ample divisors in toric geometry.

References

Fulton, W. (1993). Toric Varieties. Springer-Verlag.
Cox, D., Little, J., & Schenck, H. (2011). Toric Varieties. Springer-Verlag.

Introduction

In our previous article, we discussed two characterizations of ample divisors in toric geometry, as defined by Fulton. In this article, we will provide a Q&A section to help clarify any doubts or questions that readers may have.

Q: What is a toric variety?

A: A toric variety is an algebraic variety that is invariant under the action of a torus. In other words, a toric variety is a variety that has a torus as a group of automorphisms.

Q: What is a fan in toric geometry?

A: A fan in toric geometry is a finite collection of cones in a lattice that satisfy certain properties. In other words, a fan is a collection of cones that are "compatible" with each other.

Q: What is a strictly convex function on a fan?

A: A strictly convex function on a fan is a function that is convex on each cone of the fan and is a piecewise linear function on the cones of the fan. In other words, a strictly convex function on a fan is a function that is "smooth" on each cone and has a "nice" behavior on the cones.

Q: What is the first characterization of an ample divisor on a toric variety?

A: The first characterization of an ample divisor on a toric variety is that a divisor $D$ on a toric variety $X$ is ample if and only if the function $\psi_D$ associated with $D$ is strictly convex on the fan $\Delta$ of $X$ .

Q: What is the second characterization of an ample divisor on a toric variety?

A: The second characterization of an ample divisor on a toric variety is that a divisor $D$ on a toric variety $X$ is ample if and only if for any two points $x, y \in X$ , the line segment joining $x$ and $y$ does not lie in the graph of $\psi_D$ on the cone $\sigma \in \Delta$ associated with $D$ .

Q: What is the relationship between the two characterizations of an ample divisor on a toric variety?

A: The two characterizations of an ample divisor on a toric variety are equivalent. In other words, a divisor $D$ on a toric variety $X$ is ample if and only if the function $\psi_D$ associated with $D$ is strictly convex on the fan $\Delta$ of $X$ and for any two points $x, y \in X$ , the line segment joining $x$ and $y$ does not lie in the graph of $\psi_D$ on the cone $\sigma \in \Delta$ associated with $D$ .

Q: What are some applications of ample divisors on toric varieties?

A: Ample divisors on toric varieties have many applications in algebraic geometry and other areas of mathematics. Some examples include:

Constructing algebraic varieties: Ample divisors can be used to construct algebraic varieties with specific properties.
Computing invariants: Ample divisors can be used to compute invariants of algebraic varieties, such as their dimension and degree.
Solving equations: Ample divisors can be used to solve equations on algebraic varieties.

Q: What are some open problems in the study of ample divisors on toric varieties?

A: There are many open problems in the study of ample divisors on toric varieties. Some examples include:

Characterizing ample divisors: There is still a need to develop a complete characterization of ample divisors on toric varieties.
Computing invariants: There is still a need to develop efficient algorithms for computing invariants of algebraic varieties using ample divisors.
Solving equations: There is still a need to develop efficient algorithms for solving equations on algebraic varieties using ample divisors.

Conclusion

In this article, we have provided a Q&A section to help clarify any doubts or questions that readers may have about the two characterizations of ample divisors in toric geometry, as defined by Fulton. We hope that this article has been helpful in providing a better understanding of this topic.

References

Fulton, W. (1993). Toric Varieties. Springer-Verlag.
Cox, D., Little, J., & Schenck, H. (2011). Toric Varieties. Springer-Verlag.