Let $R$ be a commutative ring with identity. Is there any characterization for invertible elements of $R[x,x^{-1}]$ ?



Laurent Polynomials
I have read about this somewhere. I think it was as follows: $\sum\_{i=-n}^n a\_ix^i \in R[x,x^{-1}]$ is invertible iff $\sum a\_i^2$ is invertible in $R$ and for all $i \not = j$, $a\_ia\_j$ is nilpotent. 


