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Topic: Re: Elementary functions
Replies: 0

 Chih-Han sah Posts: 75 Registered: 12/3/04
Re: Elementary functions
Posted: Aug 1, 1995 10:57 PM

There are two definitions for *elementary functions*.
The first one appears to be *elementary*, but leads to all sorts
of "guesses". The second one is not *elementary* in that it involves
some "non-elementary concepts" in abstract algebra.

First one is a *practical one*. A function of a finite
number of real (or complex) variaible is called *elementary* if
it is the *composite* built up from a finite number of the following
types of functions:

*algebraic*, exponential, logarithmic, trignometric,
or inverse trigonometric.

Roughly, they comprise the most common types of function in
elementary calculus. The difficulty of this definition is that elementary
functions need not be single-valued, nor everywhere defined. There is
also something that had not been said: where is the meaning of *composite*.
For example: is sin x + arc tan y + sqrt( z^3 + ln(u)/exp(v/w))
an "elementary function" or not.

Second one is more systematic and due to J. Liouville (1838). He
defined elementary function by assigning a *class* to each of them.

Class 0 comprised of all *algebraic functions* of a finite number
of complex variables.

This can be concretely defined via the concept of a *field* in
algebra. Namely, begin with the complex numbers C and form the field
C(x_1, ..., x_n, ....) of rational functions in a countably infinite
set of variables x_i. Use the general theory of fields to construct
the algebraic closured C_0 of this field. Its elements are the
algebraic functions of class 0.

To go to elementary functions of class 1, we introduce functions
exp(z) and ln(z).

Now, inductively, we assume that elementary functions of class at most
n-1 has been defined (n a positive integer), let g(t), g_j(w_1,...,w_n)
be a finite collection of elementary functions of class at most 1,
j = 1, ..., m, and let f(z_1,...,z_m) [same m] be an elementary function
of class at most n-1. Then the composite functions: g(f(z_1,...,z_m) and
f(g_1(w_1,...,w_n),...,g_m(w_1,...,w_n) (and only such are called elementary
functions of class at most n.

While it is true that the derivative of an elementary function
is again an elementary function, it is not true that the integral of an
elementary function is necessarily elementary. Similarly, the "inverse
function" of an elementary function need not be elementary. The entire
subject is now a part of "differential field theory"--a part of algebra.
Liouville carried out a deep and detailed analysis where the integral of
an elementary function is again an elementary function. What happened
prior to Liouville was that mathematicians spoke of "non-elementary functions"
without any "agreement" and controversies abound. The main point was that
the integrals of *elementary functions* are very interesting. This is
the *inverse* process to *differentation*. Many physical problems began
with elementary functions but ends up asking about the knowledge of the
integral of these elementary functions.

In the rush to de-emphasize the techniques of integration on the
grounds that integrals can be evaluated using tools like the graphing
calculators, and at the same time of trying to emphasize the *concept*,
over *blind calculations*, one runs into the paradox: we are throwing
away the basis of many of the important concepts we are trying to teach.

Han Sah, sah@math.sunysb.edu

P.S. While Euler's formula and the Konigsburg Bridge Problem were often