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Determining the relationship to elementary functions?


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#1 SFNQuestions

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Posted 24 January 2017 - 02:23 AM

How does one determine with 100% certainty that a certain function can't ever possibly be put into terms of elementary functions, or really any arbitrary list of functions?

Take the gamma function for instance, \Gamma(x). I am fairly confident the gamma function cannot be put into terms of elementary functions, but on the other hand, I've never ever seen anything remotely resembling a proof of that statement or any statement of that nature about any special function. How would I determine it can't be put into terms of other non-special functions or a mix of special functions and elementary functions? 


Edited by SFNQuestions, 24 January 2017 - 02:25 AM.

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#2 studiot

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Posted 24 January 2017 - 02:17 PM

How does one determine with 100% certainty that a certain function can't ever possibly be put into terms of elementary functions, or really any arbitrary list of functions?

Take the gamma function for instance, \Gamma(x). I am fairly confident the gamma function cannot be put into terms of elementary functions, but on the other hand, I've never ever seen anything remotely resembling a proof of that statement or any statement of that nature about any special function. How would I determine it can't be put into terms of other non-special functions or a mix of special functions and elementary functions? 

 

 

I take it this question is related to your other one.

 

The sort of proof you are looking for is known as an existence and uniqueness theorem, of which there are several each pertaining to a particular area of mathematics.

 

It looks as though you are studying what is known as the functions of a real variable.

 

Even in this limited area the E&U theorems involve some highly abstract maths which fills large textbooks.

 

A few 'standards' are

 

Hurewicz  Dimension Theory

 

Hobson The Theory of Functions of a Real Variable (2 vols)

 

Titchmarsh The Theory of Functions

 

Burkhill A second Course in Mathematical Analysis

 

Graves Theory of Functions of Real Variables

 

Verblunsky  An introduction to the Theory of Functions of a Real Variable

 

Littlewood The Elements of the Theory of Real Functions

 

 

Some of these are rather old.


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#3 SFNQuestions

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Posted 27 January 2017 - 02:44 AM

 

 

I take it this question is related to your other one.

 

The sort of proof you are looking for is known as an existence and uniqueness theorem, of which there are several each pertaining to a particular area of mathematics.

 

It looks as though you are studying what is known as the functions of a real variable.

 

Even in this limited area the E&U theorems involve some highly abstract maths which fills large textbooks.

 

A few 'standards' are

 

Hurewicz  Dimension Theory

 

Hobson The Theory of Functions of a Real Variable (2 vols)

 

Titchmarsh The Theory of Functions

 

Burkhill A second Course in Mathematical Analysis

 

Graves Theory of Functions of Real Variables

 

Verblunsky  An introduction to the Theory of Functions of a Real Variable

 

Littlewood The Elements of the Theory of Real Functions

 

 

Some of these are rather old.

I mean I appreciate you trying to help, but I don't see any indication my question is answered. I'm not referring to a uniqueness theorem as a solution to a functional equation, this is different. And it doesn't need to be limited to functions of a real variable. What I mean is: how can you test different functions against another function to determine if one function can somehow be put into terms of another function at its simplest level? For instance, take the cosine function. For the longest time, no one had any idea about complex exponents, people pre-hand made the assumption that you simply couldn't represent the cosine function in a fundamentally different way as a function of a variable, only to later discover it could be represented as functions of complex variables. That's the mistake I want to avoid. I may have a function or Taylor series that may not look like anything I recognize, but is it its own fundamental combination of operators? Or is it a complicated culmination of pre-existing functions? That's what I want to determine. 


Edited by SFNQuestions, 27 January 2017 - 03:39 AM.

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#4 SFNQuestions

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Posted 17 April 2017 - 01:29 AM

I know there's an answer to this, because if there wasn't, every single person who ever said "cannot be put into terms of elementary functions" is automatically discretited, so I know they definitely must have done something to verify that. 


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