Find the period of Cosine of Quadratic function

Click For Summary

Discussion Overview

The discussion centers on the periodicity of the function defined as y2 = cos(ax^2 + b), where a and b are constants. Participants explore whether this function is periodic with respect to x, examining both theoretical reasoning and graphical examples.

Discussion Character

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

Main Points Raised

  • One participant questions the periodicity of y2, suggesting that the wavelengths decrease as x increases, and proposes a method to prove non-periodicity by assuming a period p and finding a contradiction.
  • Another participant attempts to derive a relationship showing that T, the period, is a function of x, concluding that the function is not periodic.
  • A specific example is provided where a participant claims that for certain values of a and b, the function appears periodic over a specific range of x, despite the earlier conclusions about non-periodicity.
  • Concerns are raised about the accuracy of numerical computations for sine and cosine functions at larger arguments, suggesting that observed periodicity might be influenced by numerical inaccuracies.

Areas of Agreement / Disagreement

Participants express differing views on the periodicity of the function, with some arguing it is not periodic while others present examples that suggest it may appear periodic under certain conditions. The discussion remains unresolved.

Contextual Notes

Participants note that the behavior of the function may depend on the values of a and b, and that numerical precision can affect the perceived periodicity of the function.

cybershakith
Messages
2
Reaction score
0
Hi all,

Hope some here can help me with this math problem.

Given,
y1 = ax^2 + b.
y2 = cos (y1).
where a and b are constants. Is y2 periodic with respect to x.? Visually using example grpah, seems to be periodic. How do u find the exact period of such a function?

Thanks in advance.

regards,
cybershakith
 
Mathematics news on Phys.org
From the graph you should see that it is not periodic, the wavelengths are decreasing as x grows larger. To prove this, assume it has period p, and find an x such that cos(ax^2+b) =/= cos(a(x+p)^2+b). Note that this only works when a is not 0. If a is zero the function is constant and trivially periodic.
 
I tried something along those lines.

cos (a*x^2 + b) = cos (a*(x+T)^2 + b)

Hence,
a*x^2 + b + 2* PI*k = a*(x+T)^2 + b, where is k is an integer.

Which reduces to,

2*PI*k = a*2*x*T + a * T^2

So T is function of x.

So the function is not periodic.

But let's take an example,
y = Cos (2*PI*ax^2 + 2*PI*b)
where is a =0.01277777778 and b = 255.5555556;

From plotting this graph, it seems like the y values are peridoc over x = 900.
So how does it happen?

for x =0;
y = Cos (2*PI*ax^2 + 2*PI*b) = Cos 2*PI * 255.5555556;

for x = 900;
y = Cos (2*PI*ax^2 + 2*PI*b) = Cos 2*PI*( 255.5555556 + 0.01277777778*900^2 ) = Cos 2*PI*(10605.5555556);


for x =11;
y = Cos (2*PI*ax^2 + 2*PI*b) = Cos 2*PI*( 255.5555556 + 0.01277777778*11^2 ) = Cos 2*PI*(257.10166671138);

for x = 911;
y = Cos (2*PI*ax^2 + 2*PI*b) = Cos 2*PI*( 255.5555556 + 0.01277777778*911^2 ) = Cos 2*PI*(10860.10166855538); //slight difference due to lack of precision.

This is true for all x, it seems.

this is because fractional part of ax^2 and a(x+T)^2 terms are the same.

So is it periodic?
 
I haven't checked out your examples in detail, I'd just like to add that the common way of calculating sin/cos on a computer becomes less accurate the further away from 0 the argument is. Thus it's possible that the observed period is due to numerical inaccuracies.
 

Similar threads

  • · Replies 139 ·
5
Replies
139
Views
12K
Replies
8
Views
2K
  • · Replies 4 ·
Replies
4
Views
5K
  • · Replies 2 ·
Replies
2
Views
2K
Replies
2
Views
2K
  • · Replies 14 ·
Replies
14
Views
5K
Replies
3
Views
2K
  • · Replies 6 ·
Replies
6
Views
2K
Replies
17
Views
3K
  • · Replies 16 ·
Replies
16
Views
4K