Borel-Moore Homology

Definition

Borel-Moore homology is a type of homology theory that extends the classical notion of homology to locally compact spaces, particularly focusing on non-compact and singular spaces. It effectively captures topological properties of spaces that can be approached via compact subsets, making it especially useful in intersection theory and the study of varieties. This theory allows for duality results similar to those in classical homology, particularly relating to Lefschetz duality, which connects the homology of a space with the cohomology of its complement.

5 Must Know Facts For Your Next Test

1. Borel-Moore homology is particularly well-suited for spaces that are not compact, as it focuses on the behavior at infinity and can use compact subsets to extract meaningful topological information.
2. The Borel-Moore homology groups are defined using singular chains, but with an adjustment for non-compactness, considering chains with support in compact sets.
3. It is often denoted as \( H^n_{BM}(X) \) for a space \( X \) and provides a way to compute homological invariants even when traditional methods fail due to non-compactness.
4. One of the key applications of Borel-Moore homology is in the field of algebraic geometry, particularly in studying projective varieties and their properties.
5. Borel-Moore homology preserves many duality results similar to classical homology theories, making it a valuable tool in understanding relationships between spaces and their complements.

Review Questions

• How does Borel-Moore homology adapt classical homology concepts to accommodate locally compact spaces?
  • Borel-Moore homology adapts classical concepts by focusing on chains that have compact support, allowing the treatment of non-compact spaces. This adaptation helps in extracting topological information from these spaces by relating their structure to that of compact subsets. As such, it provides tools for analyzing spaces where traditional homology theories may not apply due to lack of compactness.
• Discuss how Borel-Moore homology connects with Lefschetz duality and what implications this has for understanding spaces.
  • Borel-Moore homology connects with Lefschetz duality by establishing a relationship between the homology groups of a space and the cohomology groups of its complement. This connection allows mathematicians to understand how properties of a space can inform those of its surrounding environment. Such insights are crucial for deeper explorations in algebraic topology and geometry, as they help reveal underlying structures in complex topological settings.
• Evaluate the significance of Borel-Moore homology in modern mathematics and its impact on intersection theory.
  • Borel-Moore homology holds significant importance in modern mathematics by providing tools to analyze varieties and their intersections in algebraic geometry. Its ability to handle non-compact spaces expands the scope of intersection theory, allowing for a more comprehensive understanding of how different geometric objects relate. The duality results obtained through this theory enhance our grasp on complex interactions within geometric frameworks, making it a powerful approach for contemporary mathematical research.

Related terms

Locally Compact Space: A topological space that is locally compact if every point has a neighborhood that is compact.

Intersection Theory:

A branch of algebraic geometry that studies the intersection of subvarieties and how their dimensions relate.

Lefschetz Duality: A theorem that relates the homology and cohomology groups of a space with its complement, providing insights into their duality.