Split exact sequence
# Split exact sequence

In mathematics, a split exact sequence is a short exact sequence in which the middle term is built out of the two outer terms in the simplest possible way.

## Equivalent characterizations

A short exact sequence of abelian groups or of modules over a fixed ring, or more generally of objects in an abelian category

    $0\to A\mathrel {\stackrel {a}{\to }} B\mathrel {\stackrel {b}{\to }} C\to 0$

is called split exact if it is isomorphic to the exact sequence where the middle term is the direct sum of the outer ones:

    $0\to A\mathrel {\stackrel {i}{\to }} A\oplus C\mathrel {\stackrel {p}{\to }} C\to 0$

The requirement that the sequence is isomorphic means that there is an isomorphism $f:B\to A\oplus C$ such that the composite $f\circ a$ is the natural inclusion $i:A\to A\oplus C$ and such that the composite $p\circ f$ equals b. This can be summarized by a commutative diagram as:

The splitting lemma provides further equivalent characterizations of split exact sequences.

## Examples

A trivial example of a split short exact sequence is

    $0\to M_{1}\mathrel {\stackrel {q}{\to }} M_{1}\oplus M_{2}\mathrel {\stackrel {p}{\to }} M_{2}\to 0$

where $M_{1},M_{2}$ are R-modules, $q$ is the canonical injection and $p$ is the canonical projection.

Any short exact sequence of vector spaces is split exact. This is a rephrasing of the fact that any set of linearly independent vectors in a vector space can be extended to a basis.

The exact sequence $0\to \mathbf {Z} \mathrel {\stackrel {2}{\to }} \mathbf {Z} \to \mathbf {Z} /2\to 0$ (where the first map is multiplication by 2) is not split exact.

## Related notions

Pure exact sequences can be characterized as the filtered colimits of split exact sequences.

