Looking for S-shaped function with range 0 to 1 (but not asymptotic)

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Discussion Overview

The discussion revolves around the search for an S-shaped function that transitions from 0 to 1 without being asymptotic, specifically focusing on functions that have values and derivatives approaching 0 at both ends of the interval. The scope includes theoretical exploration and mathematical reasoning regarding the properties of such functions.

Discussion Character

  • Exploratory
  • Mathematical reasoning

Main Points Raised

  • One participant mentions the function Exp(-1 / x^2) as an example that grows from 0 at x = 0+ to 1 as x approaches infinity, noting its derivatives also approach 0 at the endpoints.
  • Another participant suggests taking the integral of any smooth function with compact support as a potential method to construct the desired function.
  • A later reply references the bump function Exp(-1 / (1-x^2)) and inquires about its integral, expressing difficulty in finding a closed form for it.
  • One participant asserts that while there is no closed form for the integral of the bump function, closed forms exist for all its derivatives.
  • There is a suggestion to combine two functions of the form Exp(-1 / x^2) and 1-Exp(-1 / x^2) to create the desired function, though uncertainty remains about whether this would maintain defined derivatives.

Areas of Agreement / Disagreement

Participants express varying approaches to constructing the S-shaped function, with no consensus on a definitive method or solution. Multiple competing views and uncertainties remain regarding the properties and integrals of the proposed functions.

Contextual Notes

Limitations include the lack of a closed form for the integral of the bump function and the unresolved question of whether combining certain functions would yield a valid S-shaped function with the desired properties.

Swamp Thing
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At 02:08, this video shows a function that grows from exactly 0 at input x = 0+, up to 1 at ##x=\infty##.
Its value and all its derivatives approach 0 as x -> 0. The function is Exp(-1 / x^2).

www.youtube.com/watch?v=Wwg_15a0DJo&t=146s

Q. : What function would have its value and all derivatives = 0 at 0+, then grow with x and attain a value of unity at x -> 1- with all derivatives also tending to zero as x -> 1- ?
 
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Thanks, that's some good info.

They give the example of the bump function Exp( - 1 / (1-x^2)).
What is the integral of that? I'm not good at integrating expressions, so I tried Mathematica, which also couldn't come up with a closed form.

It would be nice to have a closed form for the final S-shaped function.
 
There is no closed form of that, but you have a closed form for all derivatives.

You can try to splice together two functions of the shape of Exp(-1 / x^2) and 1-Exp(-1 / x^2), but I'm not sure if you can make all derivatives stay defined.
 

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