Continuous Function Outgrowing Polynomial but Not Exponential

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Discussion Overview

The discussion centers around the existence of a continuous function that grows faster than polynomial functions but slower than exponential functions. Participants explore the conditions under which such a function might exist, examining the relationships between growth rates of different types of functions.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant poses the question of whether a continuous function can be found such that \(\frac{x^n}{f(x)} \to 0\) and \(\frac{f(x)}{a^x} \to 0\) for all positive real \(n\) and \(a\).
  • Another participant suggests that by taking logarithms of polynomial and exponential functions, one can analyze their growth rates through functions \(g_1(x) = C_1 \ln x\) and \(g_2(x) = C_2 x\), proposing that a function can be found that grows faster than \(g_1\) and slower than \(g_2\).
  • A subsequent reply identifies \(x^{\sqrt{x}}\) as an example of a function that fits the criteria of outgrowing polynomial growth but not exponential growth.
  • Additional examples are proposed, including \(e^{\sqrt{x}}\) and \(e^{x/\ln{x}}\), suggesting that multiple functions may satisfy the initial conditions.
  • One participant expresses confidence that many such functions can be identified, reinforcing the idea that the search for these functions is fruitful.

Areas of Agreement / Disagreement

Participants appear to agree that such functions can exist, with multiple examples provided. However, the discussion does not reach a consensus on a definitive characterization or a singular function that meets the criteria.

Contextual Notes

The discussion relies on the assumptions regarding the growth rates of functions and the definitions of polynomial and exponential growth. The specific conditions under which the proposed functions operate are not fully resolved.

disregardthat
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Does there exist a continuous function which outgrows polynomial growth, but not exponential growth?

I.e. does a there exist a continuous function f such that \frac{x^n}{f(x)} \to 0 and \frac{f(x)}{a^x} \to 0 for all positive real n and a?
 
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Yes. Look at it this way: you take logs of the polynomial and the exponential, you get g_1(x) = C_1 \ln x and g_2(x) = C_2 x. Can you find a function that grows faster than g_1 and slower than g_2 for all C? Clearly you can, because ln grows extremely slowly.
 
Thanks,

x^{\sqrt{x}} is such a function.
 
Or e^{\sqrt{x}}, or e^{x/\ln{x}}.
 
I'm sure we can find many as you pointed out.
 

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