  Let $X$ be an integral affine scheme of characteristic $p>0$, and $\sigma $ a
non-identity automorphism of $X$. If $\sigma $ is $\textit{exponential}$, i.e.,
induced from a ${\bf G}_a$-action on $X$, then $\sigma $ is obviously of order
$p$. It is easy to see that the converse is not true in general. In fact, there
exists $X$ which admits an automorphism of order $p$, but admits no non-trivial
${\bf G}_a$-actions. However, the situation is not clear in the case where $X$
is the affine space ${\bf A}_R^n$, because ${\bf A}_R^n$ admits various ${\bf
G}_a$-actions as well as automorphisms of order $p$.
  In this paper, we study exponentiality of automorphisms of ${\bf A}_R^n$ of
order $p$, where the difficulty stems from the non-uniqueness of ${\bf
G}_a$-actions inducing an exponential automorphism. Our main results are as
follows.
  (1) We show that the triangular automorphisms of ${\bf A}_R^n$ of order $p$
are exponential in some low-dimensional cases.
  (2) We construct a non-exponential automorphism of ${\bf A}_R^n$ of order $p$
for each $n\ge 2$. Here, $R$ is any UFD which is not a field.
  (3) We investigate the ${\bf G}_a$-actions inducing an elementary
automorphism of ${\bf A}_R^n$.
