Formal power series exponential
= Formal power series exponential
For a <formal power series> $A$ with zero constant term over a characteristic-zero <field>, its exponential is $\exp A=\sum_{k\ge0}A^k/k!$. Every coefficient is a finite sum because $A^k$ starts in degree at least $k$. In finite characteristic, truncated computations require the factorial denominators that occur to be invertible.