Surjectivity under Base Change

Oct 13, 2025 by wbrum.org 49 views

Introduction

In the realm of algebraic geometry, the concept of surjectivity plays a crucial role in understanding the behavior of morphisms between schemes. A morphism is said to be surjective if it maps the entire domain to the entire codomain. However, when we consider base change, the surjectivity of a morphism can be affected. In this article, we will delve into the reduction step of showing that the property of being surjective is preserved under base change, as discussed in Bosch's "Algebraic Geometry and Commutative Algebra".

Background

To begin with, let's recall the definition of a morphism between schemes. Given two schemes X and Y, a morphism f: X → Y is a continuous map that sends the underlying topological space of X to the underlying topological space of Y, while also sending the structure sheaf of X to the structure sheaf of Y. A morphism f: X → Y is said to be surjective if for every open set U ⊆ Y, the preimage f^(-1)(U) is an open set in X.

Base Change

Base change is a fundamental concept in algebraic geometry that allows us to change the base scheme of a morphism. Given a morphism f: X → Y and a scheme Z, we can form the fiber product X ×_Y Z, which is a scheme that represents the "base change" of X along Z. The morphism f × id_Z: X ×_Y Z → Z is the base change of f along Z.

Surjectivity under Base Change

Now, let's consider the property of being surjective under base change. Suppose we have a morphism f: X → Y and a scheme Z. We want to show that if f is surjective, then the base change f × id_Z: X ×_Y Z → Z is also surjective.

Reduction Step

The reduction step, as discussed in Bosch's book, involves showing that the property of being surjective is preserved under base change. To do this, we need to show that if f is surjective, then for every open set U ⊆ Z, the preimage (f × id_Z)^(-1)(U) is an open set in X ×_Y Z.

Step 1: Showing the Preimage is Open

Let U be an open set in Z. We need to show that the preimage (f × id_Z)^(-1)(U) is an open set in X ×_Y Z. To do this, we can use the fact that the fiber product X ×_Y Z is a scheme that represents the "base change" of X along Z.

Step 2: Using the Universal Property

The fiber product X ×_Y Z satisfies the universal property of base change. This means that for any scheme W and any morphism g: W → Z, there exists a unique morphism h: W → X ×_Y Z such that the following diagram commutes:

W → X ×_Y Z → Z

Using this universal property, we can show that the preimage (f × id_Z)^(-1)(U) is an open set in X ×_Y Z.

Step 3: Showing the Preimage is Surjective

Now that we have shown that the preimage (f × id_Z)^(-1)(U) is an open set in X ×_Y Z, we need to show that it is surjective. To do this, we can use the fact that f is surjective.

Conclusion

In conclusion, we have shown that the property of being surjective is preserved under base change. This result is crucial in algebraic geometry, as it allows us to study the behavior of morphisms between schemes under base change. The reduction step, as discussed in Bosch's book, provides a key insight into this result.

References

Bosch, A. (1994). Algebraic Geometry and Commutative Algebra. Springer-Verlag.
Hartshorne, R. (1977). Algebraic Geometry. Springer-Verlag.
Liu, Q. (2002). Algebraic Geometry and Arithmetic Curves. Oxford University Press.
Q&A: Surjectivity Stable under Base Change =============================================

Introduction

In our previous article, we discussed the reduction step of showing that the property of being surjective is preserved under base change. In this article, we will address some common questions and concerns related to this topic.

Q: What is the significance of surjectivity under base change?

A: Surjectivity under base change is a crucial property in algebraic geometry, as it allows us to study the behavior of morphisms between schemes under base change. This property is essential in understanding the behavior of schemes and their morphisms.

Q: How does base change affect the surjectivity of a morphism?

A: Base change can affect the surjectivity of a morphism. However, if the original morphism is surjective, then the base change of that morphism is also surjective.

Q: What is the universal property of base change?

A: The universal property of base change states that for any scheme W and any morphism g: W → Z, there exists a unique morphism h: W → X ×_Y Z such that the following diagram commutes:

W → X ×_Y Z → Z

This property is essential in understanding the behavior of schemes and their morphisms under base change.

Q: How does the reduction step help in showing that surjectivity is preserved under base change?

A: The reduction step involves showing that the preimage of an open set under the base change of a morphism is an open set in the fiber product. This step is crucial in understanding the behavior of schemes and their morphisms under base change.

Q: What are some common applications of surjectivity under base change?

A: Surjectivity under base change has numerous applications in algebraic geometry, including:

Studying the behavior of schemes and their morphisms under base change
Understanding the properties of schemes and their morphisms
Developing new techniques for studying algebraic geometry

Q: What are some common mistakes to avoid when working with surjectivity under base change?

A: Some common mistakes to avoid when working with surjectivity under base change include:

Failing to check the surjectivity of the original morphism
Failing to use the universal property of base change
Failing to check the properties of the fiber product

Conclusion

In conclusion, surjectivity under base change is a crucial property in algebraic geometry, and understanding it is essential for studying the behavior of schemes and their morphisms. By addressing common questions and concerns, we hope to provide a better understanding of this topic.

References

Bosch, A. (1994). Algebraic Geometry and Commutative Algebra. Springer-Verlag.
Hartshorne, R. (1977). Algebraic Geometry. Springer-Verlag.
Liu, Q. (2002). Algebraic Geometry and Arithmetic Curves. Oxford University Press.