Fonction smaller than iterated logs, but not constant

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Discussion Overview

The discussion centers around the search for a continuous and increasing function f(x) that tends to infinity as x approaches infinity, while remaining smaller than the k-th iterated logarithm logk(x) for all k ≥ 1. Participants explore the properties and potential constructions of such a function, addressing challenges related to its definition and behavior.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant seeks a function f(x) that is continuous, increasing, and tends to infinity, while being eventually smaller than logk(x) for all k ≥ 1.
  • Another participant proposes defining an increasing function h(x) on a specific interval and suggests extending it to larger intervals, questioning the feasibility of this extension and its implications.
  • A participant raises the idea of a step function that is increasing but piecewise constant, which grows slower than both the logarithm and iterated logarithm.
  • There is a clarification regarding the conditions under which a function can be smaller than logk(x), with one participant asserting that no function can satisfy a specific interpretation of the condition.
  • One participant challenges the assertion that logk(x) tends to 0 as k tends to infinity, indicating a disagreement on this point.

Areas of Agreement / Disagreement

Participants express differing views on the properties of logk(x) and the conditions for constructing a suitable function f(x). There is no consensus on the existence of such a function or the implications of the proposed methods.

Contextual Notes

Participants highlight the need for careful consideration of the definitions and intervals involved in extending the proposed functions, as well as the implications of the behavior of logk(x) as k increases.

fortin946
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I am looking for a continuous and increasing function f(x) which tends to infinity as x tends to infinity.
This function must have the property that it is eventually smaller than logk(x)
(the k-th iterated logarithm) for all k>=1

I have no hint how to find such a function!
One of my problems is that logk(x) tends to 0 when k tends to infinity...
then how is it possible ton find f(x) increasing?!
 
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My thoughts:

Suppose we have found an increasing function h(x) on the interval (0,x_k] such that h(x) is less than than the k th iterated log function log[k](x). Can we extend the definition of h(x) to a larger interval?

There should be some point x_(k+1) > x_k so that log[k+1] (x_(k+1)) = log[k] (x_k) + 1.0
(Eventually log[k+1] (x) must climb up well past the height where log[k] is now.)

You could extend the definition of h(x) by making it a line from h(x_k) to log[k+1](x_(k+1)).

Starting with k = 1, after N steps, you would have defined a function h(x) that is less than log[1+N] (x) on the interval from x_(1+N) to infinity.

To make the above idea precise you have to worry about a lot of details. For example, does repeating the above process extend the domain of definition to arbitrarily large values of x instead of reaching some sort of limit on the x-axis.
 
Thanks for the idea! Do we call this function log*(x) ?
 
you can always think of an step function that is increasing but piecewise constant and that grows slower than the logarithm and iterated logarithm
 
Did you want a function f such that f at some point is smaller than log^k for each k (for which log^k(x) makes sense anyway)? This is different from being smaller than f being less than log^k at some point for each k at some point (the point depending on k that is). There are no function satisfying the first condition, but the latter is already dealt with here.
 
fortin946 said:
One of my problems is that logk(x) tends to 0 when k tends to infinity...

No, it doesn't
 

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