  We prove that there is a one to one correspondence between the following
three sets: idempotent functions on a set of size $n$, complete exceptional
sequences of linear radical square zero Nakayama algebras of rank $n$ and
rooted labeled forests with $n$ nodes and height of at most one. Therefore, the
number of exceptional sequences is given by the sum
$\sum\limits^n_{j=1}\binom{n}{j}j^{n-j}$.
