Let $R$ be $p$-torsion free ($p$-prime). We know that $\mathrm{Aut}_{R/pR}((R/pR)[x]) = \mathrm{AL}_1(R/pR)$.

Can someone give me a concise description of $$\mathrm{Aut}_{R/p^nR}((R/p^nR)[x])$$ for $n> 1$? A reference would be nice.

What about multiple variables? What does $$\mathrm{Aut}_{R/p^nR}((R/p^nR)[x_1,\ldots,x_n])$$ look like?

If there isn't an easy description can one describe the normal subgroups/group homomorphisms--- in particular what homomorphisms to $(R/p^nR)$ exist? Do you know of any homomorphism to any algebraic groups? I can only seem to identify the reduction mod $p$ homomorphism to the affine linear group.

References would be stellar. This group makes me feel dumb. I can tell you that an automorphism $n=2$ is induced by a polynomial of the form $a_0x+b_0 + p a(x)$ by using $f(g(x)) =x$ implies $f'(g(x))g'(x)=1$ then studying the multiplicative units in $R[x]/p^nR[x]$ but that hasn't gotten me very far in understanding more about this group.