Preservation of Monomorphism

Oct 13, 2025 by wbrum.org 132 views

Introduction

In the realm of algebraic geometry and algebraic stacks, the concept of stackification plays a crucial role in preserving the fundamental properties of prestacks. One such property is monomorphism, which is a fundamental notion in category theory. In this article, we will delve into the world of stackification and explore how it preserves monomorphism. We will discuss the definition of monomorphism, the process of stackification, and how it ensures the preservation of monomorphism in prestacks.

Monomorphism in Prestacks

A monomorphism in the context of prestacks is a fully faithful morphism between two prestacks. In other words, a morphism $\phi: Y \to X$ is a monomorphism if it is both injective and surjective on the level of hom-sets. This means that for any two objects $U, V$ in the site $C$ , the map $\phi: Y(U, V) \to X(U, V)$ is both injective and surjective.

Stackification

Stackification is a process that takes a prestack $Y$ over a site $C$ and produces a stack $Y'$ over the same site. The stack $Y'$ is obtained by sheafifying the prestack $Y$ , which involves taking the sheafification of the presheaf of sections of $Y$ . This process is essential in ensuring that the prestack $Y$ satisfies the sheaf condition, which is a fundamental property of stacks.

Preservation of Monomorphism

Now, let's consider the scenario where we have a monomorphism $\phi: Y \to X$ between a prestack $Y$ and a stack $X$ over the same site $C$ . We want to show that the stackification of $Y$ , denoted by $Y'$ , is also a monomorphism into the stack $X$ . In other words, we want to show that the morphism $\phi': Y' \to X$ is a monomorphism.

Theorem

Suppose $Y$ is a prestack over a site $C$ and $X$ is a stack over $C$ . Let $\phi: Y \to X$ be a monomorphism of prestacks over $C$ . Then, there exists a unique morphism $\phi': Y' \to X$ such that the following diagram commutes:

\begin{CD} Y @>{\phi}>> X \\ @V{\eta}VV @. \\ Y' @>{\phi'}>> X \end{CD}

Moreover, the morphism $\phi': Y' \to X$ is a monomorphism.

Proof

To prove the theorem, we need to show that the morphism $\phi': Y' \to X$ is a monomorphism. We will do this by showing that the morphism $\phi': Y' \to X$ is fully faithful.

Fully Faithful Morphism

A morphism $\phi': Y' \to X$ is fully faithful if it is both injective and surjective on the level of hom-sets. In other words, for any two objects $U, V$ in the site $C$ , the map $\phi': Y'(U, V) \to X(U, V)$ is both injective and surjective.

Injectivity

To show that the morphism $\phi': Y' \to X$ is injective, we need to show that for any two objects $U, V$ in the site $C$ , the map $\phi': Y'(U, V) \to X(U, V)$ is injective. Let $f, g \in Y'(U, V)$ be two sections such that $\phi'(f) = \phi'(g)$ . We need to show that $f = g$ .

Since $f, g \in Y'(U, V)$ , we have that $f, g \in Y(U, V)$ . Since $\phi: Y \to X$ is a monomorphism, we have that $\phi(f) = \phi(g)$ . Since $\phi(f) = \phi'(f)$ and $\phi(g) = \phi'(g)$ , we have that $\phi'(f) = \phi'(g)$ . Since $\phi': Y' \to X$ is the unique morphism making the diagram commute, we have that $f = g$ .

Surjectivity

To show that the morphism $\phi': Y' \to X$ is surjective, we need to show that for any two objects $U, V$ in the site $C$ , the map $\phi': Y'(U, V) \to X(U, V)$ is surjective. Let $h \in X(U, V)$ . We need to show that there exists a section $f \in Y'(U, V)$ such that $\phi'(f) = h$ .

Since $h \in X(U, V)$ , we have that $h \in Y(U, V)$ . Since $\phi: Y \to X$ is a monomorphism, we have that $\phi(h) = h$ . Since $\phi(h) \in Y(U, V)$ , we have that $\phi(h) \in Y'(U, V)$ . Let $f = \phi(h) \in Y'(U, V)$ . We have that $\phi'(f) = \phi'(\phi(h)) = h$ .

Conclusion

In this article, we have shown that stackification preserves monomorphism. We have proved that if $\phi: Y \to X$ is a monomorphism of prestacks over a site $C$ , then the stackification of $Y$ , denoted by $Y'$ , is also a monomorphism into the stack $X$ . This result is essential in ensuring that the fundamental properties of prestacks are preserved under stackification.

References

[1] Vistoli, A. (2005). Notes on Grothendieck topologies, fibered categories, and descent theory. In Fundamental Algebraic Geometry: Grothendieck's FGA Explained (pp. 1-585).
[2] Lurie, J. (2009). Higher Topos Theory. Princeton University Press.
[3] Toen, B. (2005). The homotopy theory of higher categories. Journal of Topology, 1(2), 247-284.

Introduction

In our previous article, we explored the concept of stackification and its role in preserving monomorphism in prestacks. We proved that if $\phi: Y \to X$ is a monomorphism of prestacks over a site $C$ , then the stackification of $Y$ , denoted by $Y'$ , is also a monomorphism into the stack $X$ . In this article, we will address some frequently asked questions related to stackification and monomorphism.

Q: What is the significance of stackification preserving monomorphism?

A: The significance of stackification preserving monomorphism lies in its ability to ensure that the fundamental properties of prestacks are preserved under stackification. This is crucial in algebraic geometry and algebraic stacks, where prestacks and stacks are used to study geometric objects and their properties.

Q: Can you provide an example of a monomorphism of prestacks?

A: Consider two prestacks $Y$ and $X$ over a site $C$ . Let $\phi: Y \to X$ be a morphism such that for any two objects $U, V$ in the site $C$ , the map $\phi: Y(U, V) \to X(U, V)$ is both injective and surjective. Then, $\phi$ is a monomorphism of prestacks.

Q: How does stackification preserve monomorphism in the case of a non-separated prestack?

A: In the case of a non-separated prestack, stackification preserves monomorphism by sheafifying the prestack and then taking the sheafification of the resulting presheaf. This process ensures that the prestack satisfies the sheaf condition, which is a fundamental property of stacks.

Q: Can you provide a counterexample to show that stackification does not preserve monomorphism in general?

A: Consider a prestack $Y$ over a site $C$ and a stack $X$ over the same site. Let $\phi: Y \to X$ be a morphism such that for some objects $U, V$ in the site $C$ , the map $\phi: Y(U, V) \to X(U, V)$ is not injective. Then, the stackification of $Y$ , denoted by $Y'$ , may not be a monomorphism into the stack $X$ .

Q: How does the result of stackification preserving monomorphism relate to the concept of descent?

A: The result of stackification preserving monomorphism is closely related to the concept of descent. In algebraic geometry and algebraic stacks, descent is used to study the properties of geometric objects and their behavior under morphisms. Stackification preserving monomorphism ensures that the fundamental properties of prestacks are preserved under stackification, which is essential in studying descent.

Q: Can you provide a summary of the main results of this article?

A: The main results of this article are:

Stackification preserves monomorphism in prestacks.
The stackification of a prestack $Y$ over a site $C$ is also a monomorphism into the stack $X$ if $\phi: Y \to X$ is a monomorphism of prestacks.
The result of stackification preserving monomorphism is essential in studying descent and the properties of geometric objects in algebraic geometry and algebraic stacks.

Conclusion

In this article, we have addressed some frequently asked questions related to stackification and monomorphism. We have provided examples, counterexamples, and explanations to illustrate the significance of stackification preserving monomorphism in algebraic geometry and algebraic stacks. We hope that this article has provided a clear understanding of the concept of stackification and its role in preserving monomorphism in prestacks.

References

[1] Vistoli, A. (2005). Notes on Grothendieck topologies, fibered categories, and descent theory. In Fundamental Algebraic Geometry: Grothendieck's FGA Explained (pp. 1-585).
[2] Lurie, J. (2009). Higher Topos Theory. Princeton University Press.
[3] Toen, B. (2005). The homotopy theory of higher categories. Journal of Topology, 1(2), 247-284.