# nLab chain map

Contents

### Context

#### Homological algebra

homological algebra

Introduction

diagram chasing

# Contents

## Idea

A chain map is a homomorphism of chain complexes. Chain complexes with chain maps between them form the category of chain complexes.

## Definition

Let $V_\bullet, W_\bullet \in Ch_\bullet(\mathcal{A})$ be two chain complexes in some ambient additive category $\mathcal{A}$ (often assumed to be an abelian category).

###### Definition

A chain map $f : V_\bullet \to W_\bullet$ is a collection of morphism $\{f_n : V_n \to W_n\}_{n \in \mathbb{Z}}$ in $\mathcal{A}$ such that all the diagrams

$\array{ V_{n+1} &\stackrel{d^V_n}{\to}& V_n \\ \downarrow^{\mathrlap{f_{n+1}}} && \downarrow^{\mathrlap{f_{n}}} \\ W_{n+1} &\stackrel{d^W_n}{\to} & W_n }$

commute, hence such that all the equations

$f_n \circ d^V_n = d^W_{n} \circ f_{n+1}$

hold.

###### Remark

A chain map $f$ induces for each $n \in \mathbb{Z}$ a morphism $H_n(f)$ on homology groups, see prop. below. If these are all isomorphisms, then $f$ is called a quasi-isomorphism.

## Properties

### On homology

###### Proposition

For $f : C_\bullet \to D_\bullet$ a chain map, it respects boundaries and cycles, so that for all $n \in \mathbb{Z}$ it restricts to a morphism

$B_n(f) : B_n(C_\bullet) \to B_n(D_\bullet)$

and

$Z_n(f) : Z_n(C_\bullet) \to Z_n(D_\bullet) \,.$

In particular it also respects chain homology

$H_n(f) : H_n(C_\bullet) \to H_n(D_\bullet) \,.$
###### Corollary

Conversely this means that taking chain homology is a functor

$H_n(-) : Ch_\bullet(\mathcal{A}) \to \mathcal{A}$

from the category of chain complexes in $\mathcal{A}$ to $\mathcal{A}$ itself.

In fact this is a universal delta-functor.

A basic discussion is for instance in section 1.1 of

A more comprehensive discussion is in section 11 of