Additive categories

Additive categories are a specific type of category in the field of category theory, which is a branch of mathematics that deals with abstract structures and relationships between them. An additive category can be thought of as a category that has some additional structure that makes it behave somewhat like the category of abelian groups or vector spaces.

An **Abelian category** is a type of category in the field of category theory that has particular properties making it a suitable framework for doing homological algebra. The notion was introduced by the mathematician Alexander Grothendieck in the context of algebraic geometry, but it has applications across various areas of mathematics.

In category theory, an **additive category** is a type of category that has a structure allowing for the definition and manipulation of "additive" operations on its objects and morphisms. Here are the key characteristics that define an additive category: 1. **Abelian Groups as Hom-Sets:** For any two objects \( A \) and \( B \) in the category, the set of morphisms \( \text{Hom}(A, B) \) forms an abelian group.

A "biproduct" typically refers to a secondary product that is produced during the manufacturing or processing of a primary product. This term is often used in various industries, such as agriculture, food processing, and manufacturing, to describe materials or substances that are not the main focus of production but can still have value or utility. For example, in the production of cheese, whey is a biproduct that can be used in various food products or as animal feed.

In mathematics, particularly in the fields of algebraic topology and homological algebra, the term "double complex" refers to a structure that arises from a collection of elements arranged in a two-dimensional grid, where each entry can have additional structure, typically in the context of chain complexes. A double complex consists of a sequences of abelian groups (or modules) arranged in a grid.

In category theory, an **exact category** is a mathematical structure that generalizes the notion of exact sequences from abelian categories, allowing for a more flexible treatment in various contexts, including algebraic geometry and homological algebra. An exact category consists of the following components: 1. **Category**: It starts with a category \( \mathcal{E} \) that has a class of "short exact sequences" (which are typically triples of morphisms).

In category theory, an exact functor is a specific type of functor that preserves the exactness of sequences or diagrams in the context of abelian categories or exact categories. While the precise definition can depend on the context, here are some key points about exact functors: 1. **Preservation of Exact Sequences:** An exact functor \( F: \mathcal{A} \to \mathcal{B} \) between abelian categories preserves exact sequences.

In mathematics, particularly in the field of algebraic topology and homological algebra, an **exact sequence** is a sequence of algebraic objects (like groups, modules, or vector spaces) connected by morphisms (like group homomorphisms or module homomorphisms) such that the image of one morphism is equal to the kernel of the next. This concept is crucial because it encapsulates the idea of relationships between structures and helps in understanding their properties.

The homotopy category of chain complexes is a fundamental concept in homological algebra and derived categories. It is a way to study chain complexes (collections of abelian groups or modules connected by boundary maps) up to homotopy equivalence, rather than isomorphism.

A **pre-abelian category** is a type of category that has some properties resembling those of abelian categories, but does not satisfy all the axioms necessary to be classified as abelian. The concept of pre-abelian categories provides a framework in which one can work with structures that have some of the nice features of abelian categories without requiring all of the strict conditions.

A **preadditive category** is a type of category in the field of category theory that has structures resembling abelian groups in its hom-sets. Specifically, a preadditive category satisfies the following properties: 1. **Hom-sets as Abelian Groups**: For any two objects \(A\) and \(B\) in the category, the set of morphisms \(\text{Hom}(A, B)\) forms an abelian group.

A **quasi-abelian category** is a type of category that generalizes some of the properties of abelian categories, while relaxing certain axioms. The concept is particularly useful in the study of categories arising in homological algebra, representation theory, and other areas of mathematics.

A **semi-abelian category** is a type of category that generalizes certain concepts from abelian categories while relaxing some of their requirements. Concepts from homological algebra and category theory often find applications in semi-abelian categories, especially in settings where one wants to retain some structural properties without having a full abelian structure.