# [FOM] 605: Integer and Real Functions (Harvey Friedman)

Joe Shipman joeshipman at aol.com
Fri Aug 28 00:27:06 EDT 2015

```What do you mean "none that are holomorphic"?

Is there actually a theorem of the following form?

"there is no real function f(x) such that
(1) there is a nonempty interval (a,b) in the reals such that for all x in (a,b), f(f(x))=e^x
(2) for some open set W in the complex plane that includes  the reals in (a,b), there is a holomorphic extension of f defined on W."

-- JS

Sent from my iPhone

> On Aug 27, 2015, at 1:50 PM, Timothy Y. Chow <tchow at alum.mit.edu> wrote:
>
> Harvey Friedman wrote:
>> So what we would like to have is a TWO VARIABLE functional equation with a initial condition that uniquely determines factorial on R|>=0.
>
> One thing to try, since you have a two-variable functional equation for exponentiation, is to use the fact that the gamma function is the Mellin transform of exp(-x).  Of course then one has to come up with a "fundamental purpose" for the Mellin transform.  Perhaps the fact about the Mellin transform that is closest in spirit to what you are trying to do here is that if X and Y are two independent random variables, then the Mellin transform of their products is equal to the product of the Mellin transforms of X and Y.
>
>> QUESTION: Find some natural classes of functions, X, such that for all
>> f in X there is a (unique) g in X such that f = gog.
>>
>> Horrible things seem to happen when you look at, e.g.,
>>
>> 2^n = g(g(n)) on N
>> e^x = h(h(x)) on R|>=n.
>>
>> The intuitive idea is clear. Let f(x) be the result of doing something
>> to x. Then g(x) should be doing it half way. When, there, and how does
>> this make sense?
>
> Well, there's a lot of literature on this subject, e.g.,
>
> http://reglos.de/lars/ffx.html
>
> I'm not sure that there's anything that you'd find very satisfying, though.  For example, there do exist moderately well-behaved solutions to e^x = h(h(x)) but none that are holomorphic, so it's not clear that any of the solutions are "fundamental."
>
> Tim
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```