Which series can be truncated?

  • Context: Undergrad 
  • Thread starter Thread starter HomogenousCow
  • Start date Start date
  • Tags Tags
    Series Truncated
Click For Summary

Discussion Overview

The discussion revolves around the conditions under which different types of series, specifically Taylor series and Fourier series, can be truncated for approximation purposes. Participants explore the implications of coefficient behavior and convergence in the context of these series.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant asserts that Taylor series can be truncated due to the decreasing nature of its coefficients, which are influenced by factorial growth.
  • Another participant counters that Fourier series can also be truncated, suggesting that practical applications in mathematics rely on this property.
  • A different participant clarifies that while Fourier series coefficients may not decrease in a straightforward manner, truncation is still valid.
  • Concerns are raised about the understanding of series convergence, with a participant noting that decreasing terms do not guarantee convergence and highlighting the complexity of the topic.
  • One participant explains that in physical representations of Fourier series, the squared coefficients correspond to energy distribution, implying that truncating high-frequency components is often a reasonable approximation.
  • Another participant emphasizes that the factorial behavior of Taylor series terms allows for truncation when the variable is small, reinforcing the argument for its approximation capability.

Areas of Agreement / Disagreement

Participants express differing views on the truncation of series, particularly between Taylor and Fourier series. While some agree on the truncation of Taylor series, there is no consensus on the implications and conditions surrounding the truncation of Fourier series.

Contextual Notes

Discussions include references to convergence and the behavior of coefficients, indicating a need for deeper exploration of these mathematical concepts. The conversation reflects varying levels of understanding regarding the conditions under which series can be truncated.

HomogenousCow
Messages
736
Reaction score
213
The taylor series can obviously be truncated, because the coeffecient of each series gets smaller and smaller due to the factorial.
However this is not the case with the fouriers series, there is no obvious reason why the coeffecients should get smaller and smaller.
So my question is, what kind of series can be truncated for an approximation?
 
Physics news on Phys.org
Fourier series' absolutely can be truncated; how else would applied mathematics work with them?
 
Last edited:
I did not claim that it cannot be truncated, I am simply pointing out that even though it's terms do not get successively smaller, it can still be truncated.
 
HomogenousCow said:
The taylor series can obviously be truncated, because the coeffecient of each series gets smaller and smaller due to the factorial.
That statement suggests you haven't studied the convergence of series. There are several things wrong with it - including the fact that a series doesn't necessarily converge even if the terms do "get smaller and smaller". But a thread on PF isn't the right place to explain something that takes a whole chapter in a math textbook - or even a whole textbook, depending how much detail you want to go into!

However this is not the case with the fouriers series, there is no obvious reason why the coeffecients should get smaller and smaller.
If the Fourier series represents something physical, the coefficients (squared) represent the amount of energy in each Fourier term. If the amount of energy in the system is finite, dividing the finite amount of energy into an infinite number of Fourier components means that almost all the components will have "very small" amounts of energy. Usually, the low frequency components are the only ones with "large" amounts of energy, so it's a good approximation to truncate the series and ignore all the high frequency components.
 
Yes I understand that simply getting smaller does not garuntee convergence, the point I am making here is that because of the 1/n! behavior of the successive terms, when x is small this factor "drowns out" the monomial it is multiplied to, hence then it is obvious that we can truncate the series.
 

Similar threads

  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 19 ·
Replies
19
Views
5K
  • · Replies 7 ·
Replies
7
Views
3K
  • · Replies 33 ·
2
Replies
33
Views
4K
  • · Replies 10 ·
Replies
10
Views
3K
  • · Replies 5 ·
Replies
5
Views
3K
Replies
23
Views
4K
  • · Replies 17 ·
Replies
17
Views
4K
  • · Replies 23 ·
Replies
23
Views
6K
  • · Replies 5 ·
Replies
5
Views
2K