# An integral problem?

How do you integrate $e^{e^x}$? I was able to get it down to du/(ln u) but I wasn't able to go further. Thanks!

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No, not Calculus AB level. This antiderivative is "not elementary" in the technical meaning of that term. https://en.wikipedia.org/wiki/Elementary_function

Maple says $$\int \!{{\rm e}^{{{\rm e}^{x}}}}{dx}=-{\rm Ei}_1 \left(-{{\rm e}^{x}} \right)$$ where this "exponential integral" function is $$\mathrm{Ei}_1(z) = \int_1^\infty\frac{e^{-tz}}{t}\;dt$$

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Then could you provide a non-elementary solution? I also have a friend who is interested in this. –  Ovi May 16 '13 at 16:20
This is related to one of the "non-elementary" functions which is defined by its integral, called the exponential integral Ei(x). Here's the result for $\ a^{a^x} \$ : wolframalpha.com/input/?i=integrate+a%5E%28a%5Ex%29 [Sorry... should have figured there'd be more answers coming...] –  RecklessReckoner May 16 '13 at 16:26
It's ok, thank you very much! –  Ovi May 16 '13 at 16:33
$$\int e^{e^x}dx$$ Substitute $u = e^x$. Then $du = e^x dx$
$$\int \frac{e^u}{u} du = \operatorname{Ei}(u) + C = \operatorname{Ei}(e^x) + C$$
Where $\operatorname{Ei}$ denotes the Exponential integral $\displaystyle \int_{-\infty}^{x} \frac{e^t}{t} dt$.