$e^{A t}$ can be expanded as

- $\sum_{k=0}^{\infty} \frac{\mathrm{A}^{k} t^{k}}{(k+1) !}$
- $\sum_{k=0}^{\infty} \frac{\mathrm{A}^{k} t^{k}}{k !}$
- $\sum_{k=0}^{\infty} \frac{\mathrm{A}^{k} t^{k+1}}{(k+1) !}$
- $\sum_{k=0}^{\infty} \frac{\mathrm{A}^{k} t^{k+1}}{k !}$