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I just found an article on Liouville's integrability criterion, which gave me a thought.

What makes functions like $\mathrm{Si}(x)$, $\mathrm{Ei}(x)$, $\mathrm{erfc}(x)$, etc. inherently different from $\sin{x}$, $\log{x}$, etc. ?

Related question: What is the exact definition for the elementary field, and what is its significance to this question?

Ayesha
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    There are probably a few questions listed under "Related" to the right which you would like to read. – Antonio Vargas Jan 05 '14 at 20:03
  • Well, if we think from the education perspective: every answer have to stop somewhere right? If you define a new function: solve-the-quadratic-polynomial function, then yes you can "solve" all quadratic polynomial and the teacher won't be able to find actual mathematical fault in that. So there have to be a collection of function/constant wherein your answer are required (implicitly) to come in that form, and deviation would count as a reason to lower grade. – Gina Jan 05 '14 at 20:37
  • Is your question about the difference from the perspective of constructive differential algebra, i.e. algorithms for integration in finite terms? – Bill Dubuque Jan 05 '14 at 20:41
  • ^^ It is not, though that is certainly something that merits consideration. – Ayesha Jan 05 '14 at 22:00
  • Seems related to What makes elementary functions elementary? – Sil Feb 07 '17 at 17:23

3 Answers3

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Yes, beginning from constants and the identity function, the different elementary functions are obtained by applying closure with respect to different elementary operations. Close by sum and multiplication and we get polynomials. Close by solving linear differential equations or order one, with constant coefficients, and we get the trigonometric and exponentials. Close by computing inverse function and we get logarithm and arc-trigonometric. We get those you mention after closing by integration.

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Really it is somewhat arbitrary. We don't usually include things like Si(x) etc. because they are used only in special situations. Whereas $e^x$ is everywhere -- no matter where you go you cannot escape it.

There are many, many, many special functions. So many that it is possible to take an entire course in special functions (which I once did) -- and that no doubt left out most of them.

Still, at least one eminent mathematician hated special functions and told me this story about why:

A lady was given a tour of an observatory, and the tour guide started explaining about many of the stars she could see through the telescope. When the tour was over, the lady thanked the guide and said "I'm really impressed with the work you do here. And what impresses me most of all is that you know the names of all those stars".

Well that is one opinion, and I think something of an outlier. I like special functions because they allow us to write complex things with simple notation.

Betty Mock
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The elementary functions are a special kind of closed-form expressions. If $f$ is an elementary function, the following statements are equivalent:

  • $f$ is generated from its only argument variable in a finite number of steps by performing only arithmetic operations, power functions with integer exponents, root functions, exponential functions, logarithm functions, trigonometric functions, inverse trigonometric functions, hyperbolic functions and/or inverse hyperbolic functions.

  • $f$ is generated from its only argument variable in a finite number of steps by performing only arithmetic operations, exponentials and/or logarithms.

  • $f$ is generated from its only argument variable in a finite number of steps by performing only explicit algebraic functions, exponentials and/or logarithms.

IV_
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