Find Inverse of Function w/ Floor: Tips & Tricks

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Discussion Overview

The discussion centers on finding the inverse of a function that includes a floor function. Participants explore the challenges posed by the floor function's properties and consider various approaches to express the inverse, including the use of periodicity and intervals.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions how to find the inverse of a function containing a floor function, suggesting the possibility of using a series approximation.
  • Another participant emphasizes that a function has an inverse if it is "one-to-one" and "onto," noting that the floor function is not one-to-one and thus does not have an inverse.
  • A specific function is presented as an example, where it is claimed that it does have an inverse, specifically stating that f(x) = x.
  • Participants discuss a function of the form f(x) = A.[(x/T+rho)-floor(x/T+rho)], indicating that it is periodic and lacks a global inverse, but can be inverted on intervals of length T.
  • It is noted that the fractional part of a number complicates the inversion process, as multiple inputs can yield the same output, making the inverse not qualify as a function.
  • One participant suggests defining x in terms of an integer constant to express multiple solutions for the equation, indicating a potential workaround for the inversion issue.

Areas of Agreement / Disagreement

Participants express differing views on the existence of an inverse for functions involving the floor function. While some argue that certain forms can be inverted on specific intervals, others maintain that the floor function's properties prevent a global inverse from existing.

Contextual Notes

The discussion highlights limitations related to the definitions of functions and inverses, particularly in the context of periodicity and the properties of the floor function. There are unresolved mathematical steps regarding the generalization of inverses for functions containing the floor function.

Who May Find This Useful

This discussion may be useful for those interested in mathematical functions, particularly in the context of inverses, periodic functions, and the implications of non-one-to-one mappings in mathematical analysis.

lewis198
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I was wondering how you would find the inverse of a function if it contained a floor function.
Since I can't expand it, I can't set y as a function of x, so to speak. Should I use a series approximation?
 
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* x as a function of y
 
You keep talking about "containing" the floor function without explaining what you mean by that. In general a function has an inverse if and only if it is "one-to-one" and "onto". Since the floor function is not one-to-one- its value at any number between 0 and 1 is 1- 1 the floor function itself does not have an inverse. I cannot answer your question about functions "containing the floor function" because I do not know exactly what you man by one function "containing" another. Does f(x)= [1- floor(x)]+ floor(x) "contain the floor function? Is so then that is an example that does have an inverse: it is simply f(x)= x.
 
This is what the equation looks like:

f(x)=A.[(x/T+rho)-floor(x/T+rho)]

I need to make x the subject of the equation, A,T and rho are constants.
 
lewis198 said:
This is what the equation looks like:

f(x)=A.[(x/T+rho)-floor(x/T+rho)]

I need to make x the subject of the equation, A,T and rho are constants.
Simplify it as g(x) = u - floor(u). This function is then periodic with period 1, which makes your function periodic with period T, and thus it has no global inverse. rho just moves the function around horizontally and A scales it vertically. So your function is invertible on any interval of length T, and you can add z*T to the inverse to get another solution, where z is some integer. A simple inverse for g is g-1(v) = v, where the interval is [0, 1). For your function, you would choose an interval like [0, T) and the inverse on this interval would just be the inverse of f(x) = A(x/T + rho) which is f-1(y) = T(y/A - rho).
 
Last edited:
So it is
f(x) = A . frac(x/T + rho)​
with 'frac' representing the fractional part after disregarding the integer part.

As Halls points out, frac(1.87) = frac(2.87) = frac(3.87) ... = 0.87, so it is really not possible to get the original number back, starting from 0.87. Technically, the 'inverse' does not qualify as a function.

You could, however, define x = T . (y/A - rho + C), where C is any integer, and say that you have multiple solutions for the equation y = f(x), if this is useful.
 
How did you guys see that so quickly? Amazing. God given gifts. Thanks for the help.
 
They learned the definitions!
 

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