The Number that will not be changed by derivatives

The Exponential Function

1. Exponential $e=1+1+{1 \over 2!}+{1 \over 3!}+{1 \over 4!}+{1 \over 5!}+{1 \over 6!}+{1 \over 7!}....$
2. Exponential Function $(e^{x})'=e^{x}$

so when every you meet Exponential just remind your self it will not changed by the deriivatives