Elementary Algebraic Geometry

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Ext functor

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Elementary Algebraic Geometry

Definition

The ext functor is a tool in homological algebra that measures the extent to which modules fail to be projective. Specifically, it computes the derived functors of the Hom functor, providing valuable information about the structure and properties of modules over a ring. This concept is crucial for understanding depth, regular sequences, and Cohen-Macaulay rings, as it helps analyze their homological dimensions and projective resolutions.

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5 Must Know Facts For Your Next Test

  1. The ext functor is denoted as $\text{Ext}^n(R)$ for a given ring $R$ and measures extensions of modules by looking at derived functors.
  2. When calculating $\text{Ext}^1$, one can interpret it as the group of equivalence classes of extensions of one module by another, highlighting how modules can be related to each other.
  3. The first derived functor $\text{Ext}^1(A,B)$ can also give insights into whether there are nontrivial extensions of module $B$ by module $A$.
  4. In the context of Cohen-Macaulay rings, the vanishing of certain $ ext{Ext}$ groups can imply significant geometric and algebraic properties, influencing depth and regular sequences.
  5. The use of the ext functor is pivotal when working with resolutions, particularly in determining whether a resolution is projective or injective.

Review Questions

  • How does the ext functor relate to the concepts of depth and regular sequences in module theory?
    • The ext functor helps quantify relationships between modules, particularly regarding their extensions. In terms of depth and regular sequences, the vanishing of certain $ ext{Ext}$ groups can indicate that a module has a specific depth or satisfies conditions related to regular sequences. Therefore, by analyzing these $ ext{Ext}$ groups, one can infer important structural properties about modules and their relationships within Cohen-Macaulay rings.
  • Discuss the significance of the ext functor in understanding projective resolutions and how this relates to Cohen-Macaulay rings.
    • The ext functor provides crucial insights into projective resolutions, which are sequences that allow us to decompose modules into simpler components. In Cohen-Macaulay rings, projective resolutions play a vital role in determining the depth and homological dimensions. Analyzing these resolutions with the help of the ext functor allows mathematicians to better understand how these rings behave algebraically and geometrically.
  • Evaluate how the properties of Cohen-Macaulay rings influence the behavior of ext functors and their implications for module extensions.
    • Cohen-Macaulay rings exhibit properties that simplify the behavior of ext functors. Specifically, because their depth coincides with their Krull dimension, many $ ext{Ext}$ groups will vanish under certain conditions, leading to fewer nontrivial extensions between modules. This relationship enhances our understanding of how modules over these rings interact and simplifies computations involved in module theory, paving the way for more profound insights in both algebra and geometry.
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