[FOM] 605: Integer and Real Functions (Harvey Friedman)
Timothy Y. Chow
tchow at alum.mit.edu
Fri Aug 28 21:34:47 EDT 2015
On Fri, 28 Aug 2015, Joe Shipman wrote:
> Is there f with f(f(x))=e^x which is both real analytic and monotonic
> for x>0?
Yes, Kneser's function is injective.
> Is Kneser's construction generalizable to find monotonic real analytic
> g(g(x))=f(x) and so on?
>
> Is it generalizable to find monotonic real analytic f(f(x))=2^x?
>
> What kind of conditions on such f imply uniqueness?
I don't know the answers to these questions off the top of my head. On
the page that I linked to before http://reglos.de/lars/ffx.html there is a
reference to a book "Iterative Functional Equations" by Kuczma et al. that
I know addresses these kinds of questions. Partial answers are known but
there tend to be a lot of hypotheses and conditions on the theorems,
making the subject rather difficult to digest. I'm not sure if this is
intrinsic to the subject or a reflection of our lack of ability to prove
things.
Tim
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