I know how to prove that for a commutative ring $R$, the units of the polynomial ring $R[x]$ are the polynomials $$p(x)=a_{0}+a_{1}x+\dots +a_{n}x^{n}$$ such that $a_{0}$ is a unit in $R$ and the remaining coefficients $a_{1},\dots ,a_{n}$ are nilpotent, i.e., satisfy $a_{i}^{N}=0$ for some $N$.
What can be said about the units of the polynomial ring in multiple variables, $R[x_1,\dots,x_n]$? Is there a similar characterization?
I was thinking of using that $R[x_1,\dots,x_n]= R[x_1,\dots,x_{n-1}] [x_n]$ and trying to apply induction, but Iām not sure how to proceed.