# nLab snake lemma

### Context

#### Homological algebra

homological algebra

and

nonabelian homological algebra

diagram chasing

# Contents

## Idea

A basic lemma in homological algebra: it constructs connecting homomorphisms.

## Statement

###### Lemma

Let

$\array{ && A' &\to & B' &\stackrel{p}{\to}& C' &\to & 0 \\ && \downarrow^{\mathrlap{f}} && \downarrow^{\mathrlap{g}} && \downarrow^{\mathrlap{h}} \\ 0 &\to& A &\stackrel{i}{\to} & B &\to& C }$

be a commuting diagram in an abelian category $\mathcal{A}$ such that the two rows are exact sequences.

Then there is a long exact sequence of kernels and cokernels of the form

$ker(f) \to ker(g) \to ker(h) \stackrel{\partial}{\to} coker(f) \to coker(g) \to coker(h) \,.$

Moreover

• if $A \to B$ is a monomorphism then so is $ker(f) \to ker(g)$

• if $B \to C$ is an epimorphism, then so is $coker(g) \to coker(h)$.

If $\mathcal{A}$ is realized as a (full subcategory of) a category of $R$-modules, then the connecting homomorphism $\partial$ here can be defined on elements $c' \in ker(h) \subset C'$ by

$\partial (c') := i^{-1} \,g\, p^{-1}(c') \,,$

where $i^{-1}(-)$ and $p^{-1}(-)$ denote any choice of pre-image (the total formula is independent of that choice).

###### Remark

The snake lemma derives its name from the fact that one may draw the connecting homomorphism $\partial$ that it constructs diagrammatically as follows:

## References

An early occurence of the snake lemma is as lemma (5.8) of

• D. A. Buchsbaum, Exact categories and duality, Transactions of the American Mathematical Society Vol. 80, No. 1 (1955), pp. 1-34 (JSTOR)

In

it appears as lemma 1.3.2.

A purely category-theoretic proof is given in

and in