Local cohomology

Definition

Local cohomology is a branch of algebraic topology that studies the properties of sheaves and their cohomological aspects in the vicinity of a specific subspace. It provides a way to analyze the behavior of global sections of sheaves when they are localized around a point or a closed subset, which connects well with various concepts, including cap products and sheaf cohomology.

5 Must Know Facts For Your Next Test

1. Local cohomology groups can provide information about the vanishing of sections of a sheaf outside a certain subset, highlighting their local properties.
2. They are typically denoted as $H^i_I(X)$, where $I$ is an ideal associated with a closed subset and $X$ is the topological space.
3. The local cohomology functor can be computed using Čech cohomology or derived functors, which relate to how sections behave in different parts of the space.
4. Local cohomology can be utilized to define intersection numbers in algebraic geometry through cap products, linking it to deeper geometric interpretations.
5. These groups have applications in many areas, including algebraic geometry, commutative algebra, and singularity theory, reflecting their broad relevance.

Review Questions

• How does local cohomology relate to the notion of support for sheaves?
  • Local cohomology is fundamentally concerned with understanding how sections of a sheaf behave around a closed subset, which directly ties into the concept of support. The support of a sheaf is crucial for defining local cohomology groups since these groups are defined with respect to an ideal that corresponds to the closure of this support. By analyzing local cohomology, we can gain insights into how sections vanish outside this support and understand the sheaf's local properties.
• Discuss how cap products can be understood within the framework of local cohomology.
  • Cap products serve as a way to combine elements from different cohomology groups, and in the context of local cohomology, they play a vital role in defining intersection theory. Specifically, local cohomology groups can be used to compute intersection numbers by taking a global class from one cohomology group and capping it with classes from local cohomology groups associated with specific closed subsets. This interaction highlights how local behavior influences global properties in algebraic geometry.
• Evaluate the impact of local cohomology on understanding global properties through its connection with sheaf cohomology.
  • Local cohomology has a profound impact on our understanding of global properties by bridging localized information with broader sheaf cohomological perspectives. By studying how sections behave near specific closed subsets using local cohomology, we can derive significant insights about global invariants and characteristics. This relationship not only enriches our understanding of geometric structures but also provides tools for solving complex problems in algebraic geometry and topology by allowing us to leverage local data for global applications.

Related terms

Sheaf: A sheaf is a mathematical object that systematically assigns data to open sets of a topological space, allowing for local-to-global principles in algebraic geometry and topology.

Cohomology: Cohomology is a tool used in algebraic topology to study the properties of topological spaces through algebraic invariants, enabling the computation of global properties from local data.

Support: In the context of sheaves, the support refers to the closure of the set of points where a sheaf has non-zero sections, which is crucial for understanding local cohomology.