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We know that

$$ e^x=\sum_{n=0}^{\infty}\frac{x^n}{n!},x\in \mathbb R, $$

which can be written out $$ e^x=\frac {x^0}{0!}+\frac {x^1}{1!}+\frac {x^2}{2!}+\cdots, $$

but $0^0$ isn't well defined.

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