# Expanded concept of elementary function?

After searching about why $\int e^{x^2}$ is not an elementary function, I was disappointed that I should understand about Galois theory, but then I started to think about a concept that treats elementary functions, anti-derivative of elementary functions, and operations (+-*/, composition) of those functions, which includes non-elementary functions like $\int e^{x^2}$, $\int \frac{sin(x)}x$, or $\int log(log(x))$.

I do believe that some people in past thought about that, but I simply don't know how to search.

What's the name of the type of functions that can be derived by differentiate, integrate, and composite of elementary functions? Plus, just out of curiosity, is there any function that's infinitely differentiable but not in the type of functions I asked?

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There are functions that are defined by sums and products, in addition to those defined by integrals. –  soandos Jun 2 '11 at 15:38
If I understand your boldfaced question correctly: $\int e^{x^2} dx$ is in fact nonelementary but infinitely differentiable. –  barf Jun 2 '11 at 16:31
–  Rahul Jul 2 '11 at 19:12

You might be interested in a more extreme type of non-elementary function. A function $y$ of $x$ is said to be "transcendentally transcendental" on an interval $(a,b)$ if $P(x,y,y', y'',...,y^{n})$ is not identically zero on $(a,b)$ for every positive integer $n$ and every nonzero polynomial $P$ of $n+2$ variables. In other words, $y$ doesn't satisfy any algebraic differential equation, including non-linear algebraic differential equations. None of the elementary transcendental functions (trigonometric, exponential, logarithmic) are transcendentally transcendental on any open interval, and most of the higher functions in mathematical physics aren't either (elliptic functions, Bessel functions, etc.). However, in 1887 Hölder proved that the gamma function is transcendentally transcendental, which incidentally gives a naturally occurring example of an infinitely differentiable function that is far more non-elementary than what you were asking about. (I'm not sure, but I think Hölder was also the first to formulate the property of being "transcendentally transcendental".) A nice survey paper of this topic is:

Lee Albert Rubel, "A survey of transcendentally transcendental functions", American Mathematically Monthly 96 (1989), 777-788.

Many of the older papers on this topic, including Hölder's original paper and some papers by E. H. Moore (in a 1897 paper, E. H. Moore introduced the name "transcendentally transcendental") and some papers by J. F. Ritt (1923, 1926) are in Math. Annalen, and thus are freely available on the internet. There's also a 1902 paper by Edmond Maillet in Bulletin de la Societe Mathematique de France (Vol. 30, pp. 195-201) that is freely available on the internet.

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In case anyone is interested, I had no idea this was an old question (asked 2 months ago) when I submitted my reply. For some reason this question was near the top of the list of questions when I visited StackExchange today (first visit in 2 or 3 days), and I never thought to look at the date. –  Dave L. Renfro Aug 1 '11 at 19:41
It was probably bumped by the Community user. That means it didn't have an upvoted answer, so don't worry about it: it's good for questions like that to get answers. –  Qiaochu Yuan Aug 1 '11 at 20:52
Thanks for the good answer and information! –  JiminP Aug 2 '11 at 11:03
@Dave: +1 Nice answer indeed! I was wondering if gamma function being "transcendentally transcendental" implies factorial also being "transcendentally transcendental", given that factorial is restriction of gamma function to nonnegative integers? –  Tim Jan 12 '12 at 17:35
@Tim: Since transcendentally transcendental involves satisfying a differential equation of a certain type, this seems like a non-issue (perhaps like asking if there exists a prime number that is a Hausdorff topological space). Also, restrictions of functions typically have nicer properties than the corresponding un-restricted function. For example, $\chi_{\mathbb Q}:{\mathbb R} \rightarrow {\mathbb R}$ is nowhere continuous while $\chi_{\mathbb Q}:{\mathbb Q} \rightarrow {\mathbb R}$ is constant, and $\sin$ is transcendental while its restriction to integer multiples of $\pi$ is algebraic. –  Dave L. Renfro Jan 12 '12 at 18:37