How to Find Functions for Constant Arc Length with Varying Amplitude and Period?

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Discussion Overview

The discussion revolves around finding two functions, f(n,x) and g(n,x), such that the expression f(n,x)sin(g(n,x)) maintains a constant arc length over a specified interval [a,b]. The focus is on the relationship between amplitude and period as the variable n changes.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant proposes the functions f(n,x) and g(n,x) must be defined such that the arc length remains constant, with n affecting amplitude and period.
  • Another participant suggests a trivial choice for f(n,x) and questions whether a differential equation can be established to ensure constant arc length.
  • A participant mentions having set up a differential equation and emphasizes the necessity for f(n,x) to depend on both n and x, rather than just n.
  • There is a discussion about the arc length expression derived from the functions and the implications of differentiating it.
  • One participant inquires if separation of variables has been attempted to simplify the partial differential equation into an ordinary differential equation.

Areas of Agreement / Disagreement

Participants have not reached a consensus on the methods or solutions to the problem. Multiple approaches and ideas are being explored, indicating ongoing debate and uncertainty.

Contextual Notes

The discussion involves complex mathematical expressions and assumptions that may not be fully resolved, particularly regarding the setup and solution of the differential equations involved.

JonF
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I want to find two functions f(n,x) and g(n,x) such that f(n,x)sin(g(n,x)) always has a constant arc length over some interval [a,b]. Where n increases the amplitude but decreases the period.

Any suggestions?
 
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Well, you've got the trivial choice [tex]f(n,x)=\frac{1}{\sin(g(n,x)}[/tex]

In the general case, the arc-length will be a function of "n".
Can you set up a differential equation for f and g which ensures that the arc-length is just the constant function?
 
I did set up a DE. That’s how I know they have to be f(n,x) not just f(n)

If they were…
[tex]f(x) = g(n)sin(x*p(n))[/tex]
[tex]f’(x) = g(n)p(n)cos(x*p(n))[/tex]

So arch length is:

[tex]\int_{0}^{1} \sqrt{{1+ (g(n)p(n)cos(p(n)) )^2} dx =c[/tex]

Differentiate
1+ (g(n)p(n)cos(p(n)))^2 – (1+ (g(n)p(n))^2) = 0

(g(n)p(n)cos(p(n)))^2 = (g(n)p(n))^2)

cos(p(n)) = 1

p(n) = 0

So I know if they were just functions of just n it would be trivial. For functions of x and n (possibly I will need to put a and b in there also) I know how to set up the de, no clue how to solve it.
 
Have you already tried separation of variables to transform your partial DE into an ordinary DE?

What I mean is assume g(x,n) = X(x)*N(n).
 

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