Properties of Summations and Integrals question

  • Context: Graduate 
  • Thread starter Thread starter Newbie234
  • Start date Start date
  • Tags Tags
    Integrals Properties
Click For Summary

Discussion Overview

The discussion revolves around the implications of summation and integration statements involving functions, specifically whether certain equalities hold for integrals based on summation properties and the relationship between functions given their infinite sums. The scope includes mathematical reasoning and exploration of properties of summations and integrals.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant questions whether the statement \(\sum^{\infty}_{0}f(x)=\frac{\sum^{\infty}_{0}g(x)}{\sum^{\infty}_{0}h(x)}\) implies \(\int^{\infty}_{0}f(x)=\frac{\int^{\infty}_{0}g(x)}{\int^{\infty}_{0}h(x)}\).
  • Another participant challenges the clarity of the initial statement, asking what is being summed over.
  • Further clarification is provided that the summation is over integers, and a counterexample involving a specific function \(g(x)\) is proposed to illustrate potential issues with the statements.
  • There is a discussion about whether the statements hold for continuous functions that are not piecewise, with one participant suggesting that similar counterexamples may apply.
  • One participant asserts that the first statement is false but agrees that the second statement implies asymptotic equivalence between the infinite sums of \(f(x)\) and \(g(x)\).

Areas of Agreement / Disagreement

Participants express disagreement regarding the validity of the initial statements, with some asserting that the first statement is false and others proposing counterexamples. There is no consensus on the implications of the second statement.

Contextual Notes

Participants note the importance of specifying the domain of summation and the nature of the functions involved, highlighting potential limitations in the assumptions made about the functions being continuous or piecewise.

Newbie234
Messages
4
Reaction score
0
Let's say we have the statement [itex]\sum^{\infty}_{0}f(x)=\frac{\sum^{\infty}_{0}g(x)}{\sum^{\infty}_{0}h(x)}[/itex] does this imply that
[itex]\int^{\infty}_{0}f(x)=\frac{\int^{\infty}_{0}g(x)}{\int^{\infty}_{0}h(x)}[/itex]?

Also if [itex]\sum^{\infty}_{0}f(x)=\sum^{\infty}_{0}g(x)[/itex] does this imply that [itex]f(x)=g(x)[/itex], or just that f(x)~g(x) (asymptotically equivalent)?

Thanks.
 
Last edited:
Physics news on Phys.org
Your first statement makes no sense, what are you summing over?
 
hunt_mat said:
Your first statement makes no sense, what are you summing over?

Sorry, thought it was pretty obvious, but I guess not. I didn't think it was necessary to add dx to the integrals or add that summation was over x as it was implied, and I didn't want to typeset anymore than I had to because it is new to me.
 
Newbie234 said:
Sorry, thought it was pretty obvious, but I guess not. I didn't think it was necessary to add dx to the integrals or add that summation was over x as it was implied, and I didn't want to typeset anymore than I had to because it is new to me.

Are you summing over all the integers x, or are you summing over all the reals x? If you assume the latter, it doesn't make much sense. If you're meaning the former, then consider a function like

[tex]g(x)=\left\{\begin{array}{c}1/x^2~\text{if x is an integer}\\ 0~\text{otherwise}\end{array}\right.[/tex]

that should be the basis of a counterexample for both statements.
 
micromass said:
Are you summing over all the integers x, or are you summing over all the reals x? If you assume the latter, it doesn't make much sense. If you're meaning the former, then consider a function like

[tex]g(x)=\left\{\begin{array}{c}1/x^2~\text{if x is an integer}\\ 0~\text{otherwise}\end{array}\right.[/tex]

that should be the basis of a counterexample for both statements.

Summing over integers. Also, are these statements true for continuous functions that aren't piecewise?
 
Newbie234 said:
Summing over integers. Also, are these statements true for continuous functions that aren't piecewise?

I don't think so. Similar counterexamples hold. Try to find a continuous version of my counterexample!
 
The first statement is false. But I am correct in assuming that the second statement implies that the infinite sums of f(x) and g(x) are asymptotically equivalent (that is, [itex]lim_{n->\infty}\frac{\sum^{n}_{x=0}f(x)}{\sum^{n}_{x=0}g(x)}=1[/itex]).
 
Last edited:

Similar threads

  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 3 ·
Replies
3
Views
3K
  • · Replies 5 ·
Replies
5
Views
3K
  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 3 ·
Replies
3
Views
3K
  • · Replies 9 ·
Replies
9
Views
3K
  • · Replies 5 ·
Replies
5
Views
3K
  • · Replies 3 ·
Replies
3
Views
4K
  • · Replies 25 ·
Replies
25
Views
4K