Projective Spaces and Morphisms

Oct 13, 2025 by wbrum.org 11 views

Introduction

In the realm of algebraic geometry, projective spaces play a pivotal role in understanding the properties of algebraic varieties. The projective space $\mathbb P^n$ is a fundamental object of study, and morphisms between these spaces are essential in establishing connections between different algebraic varieties. In this article, we will delve into the world of morphisms between projective spaces $\mathbb P^n$ to $\mathbb P^m$ , exploring the various types of morphisms and their properties.

Projective Spaces and Morphisms

Before we embark on our journey to explore morphisms between projective spaces, let's briefly recall the definition of a projective space. The projective space $\mathbb P^n$ over an algebraically closed field $k$ is defined as the set of all lines through the origin in the affine space $\mathbb A^{n+1}$ . In other words, $\mathbb P^n$ is the set of all points $[x_0, x_1, \ldots, x_n]$ in $\mathbb A^{n+1}$ , where not all $x_i$ are zero.

A morphism between two projective spaces $\mathbb P^n$ and $\mathbb P^m$ is a map $f: \mathbb P^n \to \mathbb P^m$ that is defined by a set of homogeneous polynomials in the coordinates of $\mathbb P^n$ . More specifically, a morphism $f: \mathbb P^n \to \mathbb P^m$ is defined by a set of homogeneous polynomials $f_0, f_1, \ldots, f_m$ in the coordinates $x_0, x_1, \ldots, x_n$ of $\mathbb P^n$ , such that the map $f: [x_0, x_1, \ldots, x_n] \mapsto [f_0(x_0, x_1, \ldots, x_n), f_1(x_0, x_1, \ldots, x_n), \ldots, f_m(x_0, x_1, \ldots, x_n)]$ is well-defined and satisfies certain properties.

Types of Morphisms

There are several types of morphisms between projective spaces, each with its own set of properties and applications. Some of the most common types of morphisms include:

Regular morphisms: A regular morphism is a morphism that is defined by a set of homogeneous polynomials of the same degree. Regular morphisms are the most common type of morphism and are used extensively in algebraic geometry.
Rational morphisms: A rational morphism is a morphism that is defined by a set of homogeneous polynomials, but the map is not necessarily defined everywhere. Rational morphisms are used to study the properties of algebraic varieties and are an essential tool in algebraic geometry.
Birational morphisms: A birational morphism is a morphism that is defined by a set of homogeneous polynomials, but the map is not necessarily defined everywhere. Birational morphisms are used to study the properties of algebraic varieties and are an essential tool in algebraic geometry.

Properties of Morphisms

Morphisms between projective spaces have several important properties that are essential in understanding their behavior. Some of the most important properties of morphisms include:

Homogeneity: A morphism is homogeneous if it is defined by a set of homogeneous polynomials. Homogeneous morphisms are the most common type of morphism and are used extensively in algebraic geometry.
Linearity: A morphism is linear if it is defined by a set of linear polynomials. Linear morphisms are used to study the properties of algebraic varieties and are an essential tool in algebraic geometry.
Injectivity: A morphism is injective if it is one-to-one. Injective morphisms are used to study the properties of algebraic varieties and are an essential tool in algebraic geometry.

Examples of Morphisms

There are several examples of morphisms between projective spaces that are used extensively in algebraic geometry. Some of the most common examples include:

The inclusion map: The inclusion map is a morphism from $\mathbb P^n$ to $\mathbb P^{n+1}$ that is defined by the map $[x_0, x_1, \ldots, x_n] \mapsto [x_0, x_1, \ldots, x_n, 0]$ .
The projection map: The projection map is a morphism from $\mathbb P^n$ to $\mathbb P^{n-1}$ that is defined by the map $[x_0, x_1, \ldots, x_n] \mapsto [x_0, x_1, \ldots, x_{n-1}]$ .
The Veronese map: The Veronese map is a morphism from $\mathbb P^n$ to $\mathbb P^{n+1}$ that is defined by the map $[x_0, x_1, \ldots, x_n] \mapsto [x_0^2, x_0x_1, \ldots, x_n^2]$ .

Conclusion

In conclusion, morphisms between projective spaces are a fundamental concept in algebraic geometry, and understanding their properties and behavior is essential in studying the properties of algebraic varieties. In this article, we have explored the various types of morphisms, their properties, and examples of morphisms between projective spaces. We hope that this article has provided a comprehensive overview of morphisms between projective spaces and has inspired readers to explore this fascinating topic further.

References

Hartshorne, R. (1977). Algebraic Geometry.
Mumford, D. (1995). Algebraic Geometry I: Complex Projective Varieties.
Griffiths, P. A., & Harris, J. (1994). Principles of Algebraic Geometry.

Introduction

In our previous article, we explored the concept of morphisms between projective spaces, including the various types of morphisms, their properties, and examples of morphisms between projective spaces. In this article, we will answer some of the most frequently asked questions about morphisms between projective spaces.

Q&A

Q: What is a morphism between projective spaces?

A: A morphism between projective spaces is a map between two projective spaces that is defined by a set of homogeneous polynomials in the coordinates of the projective spaces.

Q: What are the different types of morphisms between projective spaces?

A: There are several types of morphisms between projective spaces, including regular morphisms, rational morphisms, and birational morphisms.

Q: What is a regular morphism?

A: A regular morphism is a morphism that is defined by a set of homogeneous polynomials of the same degree.

Q: What is a rational morphism?

A: A rational morphism is a morphism that is defined by a set of homogeneous polynomials, but the map is not necessarily defined everywhere.

Q: What is a birational morphism?

A: A birational morphism is a morphism that is defined by a set of homogeneous polynomials, but the map is not necessarily defined everywhere.

Q: What are the properties of morphisms between projective spaces?

A: Morphisms between projective spaces have several important properties, including homogeneity, linearity, and injectivity.

Q: What is homogeneity in the context of morphisms between projective spaces?

A: Homogeneity in the context of morphisms between projective spaces means that the morphism is defined by a set of homogeneous polynomials.

Q: What is linearity in the context of morphisms between projective spaces?

A: Linearity in the context of morphisms between projective spaces means that the morphism is defined by a set of linear polynomials.

Q: What is injectivity in the context of morphisms between projective spaces?

A: Injectivity in the context of morphisms between projective spaces means that the morphism is one-to-one.

Q: Can you provide an example of a morphism between projective spaces?

A: Yes, one example of a morphism between projective spaces is the inclusion map, which is a morphism from $\mathbb P^n$ to $\mathbb P^{n+1}$ that is defined by the map $[x_0, x_1, \ldots, x_n] \mapsto [x_0, x_1, \ldots, x_n, 0]$ .

Q: Can you provide an example of a rational morphism between projective spaces?

A: Yes, one example of a rational morphism between projective spaces is the projection map, which is a morphism from $\mathbb P^n$ to $\mathbb P^{n-1}$ that is defined by the map $[x_0, x_1, \ldots, x_n] \mapsto [x_0, x_1, \ldots, x_{n-1}]$ .

Q: Can you provide an example of a birational morphism between projective spaces?

A: Yes, one example of a birational morphism between projective spaces is the Veronese map, which is a morphism from $\mathbb P^n$ to $\mathbb P^{n+1}$ that is defined by the map $[x_0, x_1, \ldots, x_n] \mapsto [x_0^2, x_0x_1, \ldots, x_n^2]$ .

Conclusion

In conclusion, morphisms between projective spaces are a fundamental concept in algebraic geometry, and understanding their properties and behavior is essential in studying the properties of algebraic varieties. We hope that this Q&A article has provided a comprehensive overview of morphisms between projective spaces and has inspired readers to explore this fascinating topic further.

References

Hartshorne, R. (1977). Algebraic Geometry.
Mumford, D. (1995). Algebraic Geometry I: Complex Projective Varieties.
Griffiths, P. A., & Harris, J. (1994). Principles of Algebraic Geometry.