# showing exact functors preserve exact sequences (abelian categories, additive functors, and kernels)

I’m working through Vakil’s algebraic geometry text and I’ve been stuck on Exercise 1.6.E (page 52 on http://math.stanford.edu/~vakil/216blog/FOAGjun1113public.pdf.)

Suppose that $F$ is an exact functor. Show that applying $F$ to an exact sequence preserves exactness. For example, if $F$ is covariant and $A’ \to A \to A”$ is exact, then $FA’ \to FA \to FA”$ is exact.

Here’s what I’ve been thinking:

Let the maps be denoted $f, g$ (so we have $A’ \xrightarrow{f} A \xrightarrow{g} A”$).

We know $F$ is left-exact and right-exact. To use the left-exactness of $F$, we note that $0 \to \ker f \to A \xrightarrow{g} A”$ is exact, so $0 \to F(\ker f) \to FA \xrightarrow{Fg} FA”$ is exact.

However, I’m not quite sure what to do with the $F(\ker f)$ object. (It’d be nice if $F(\ker f) = \ker Ff$ but I don’t see any reason for this to actually be true.)

#### Solutions Collecting From Web of "showing exact functors preserve exact sequences (abelian categories, additive functors, and kernels)"

We have a diagram

with exact diagonals. Applying $F$ gives a diagram

also with exact diagonals.

Now, note that
\begin{align*}
\DeclareMathOperator{Im}{Im}\Im F(f)
&= \Im\big(F(A^\prime)\to F(\Im f)\to F(A)\big) \\
&\overset{\ast}{=} \Im\big(F(\Im f)\to F(A)\big) \\
&= \DeclareMathOperator{Ker}{Ker}\Ker\big(F(A)\to F(\Im g)\big) \\
&\overset{\circledast}{=} \Ker\big(F(A)\to F(\Im g)\to F(A^{\prime\prime})\big) \\
&= \Ker F(g)
\end{align*}
where $\ast$ holds since $F(A^\prime)\to F(\Im f)$ is epi and $\circledast$ holds since $F(\Im g) \to F(A^{\prime\prime})$ is mono. Hence
$$F(A^\prime)\to F(A)\to F(A^{\prime\prime})$$
is exact.

Of course, the above argument extends to prove that an exact functor maps acyclic complexes to acyclic complexes.