Textbook error relating to uncountability?

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Discussion Overview

The discussion revolves around a claim made in a textbook regarding the relationship between discrete samples of a function and its limit as the number of samples approaches infinity. Participants explore the implications of this claim, particularly in relation to Cantor's second theorem and the nature of limits in mathematical analysis.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions the textbook's assertion that a vector of samples converges to a continuous function, suggesting it implies a bijection between natural numbers and a continuous interval, which contradicts established mathematical principles.
  • Another participant notes the vagueness of the textbook's definition of the limit and emphasizes that different types of limits exist for sequences of functions, which complicates the validity of the textbook's statement.
  • A third participant proposes that the sequence of functions must have the same domain and suggests that the textbook should have referenced step functions instead.
  • One participant draws a distinction between mathematical theory and engineering practice, arguing that the statement may hold true for bandwidth-limited signals in engineering contexts, though they criticize the textbook's explanation as lacking rigor.

Areas of Agreement / Disagreement

Participants express differing views on the validity of the textbook's claim, with some supporting the idea that it may hold under specific conditions while others challenge its accuracy. The discussion remains unresolved, with no consensus reached on the correctness of the textbook's statement.

Contextual Notes

Participants highlight the need for clearer definitions and the potential for multiple interpretations of limits in the context of sequences of functions. The discussion also touches on the implications of bandwidth limitations in engineering versus theoretical mathematics.

JerryG
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I am reading a textbook about communications systems, and just came across a part that I believe to be wrong. As shown below, it says that if you take a vector g = [g(t1) g(t2) ... g(tN)] and take the limit as the number of samples goes to infinity, the result is a continuous function of t. Wouldn't this imply a bijection between the natural numbers and a continuous interval which was disproven by cantor's second theorem?

bij1.gif


bij2.gif
 
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Informal statements are often saved from outright errors by being vague. That passage did not give a definition for lim_{N \rightarrow \infty} \mathbb{g}.

The definition of a limit of a sequence of vectors (which can be regarded as a sequence of function) cannot be deduced from the definition of the type of limit used in calculus, which deals with the limit of a single function at a given point. There are actually many different types of limits that are defined for sequences of functions. I'd have to look these up and re-study them before I could swear that none of them make the book's statement true.

You are correct that the infinite sequence of discrete time samples does not ever become an uncountable set.
 
Stephen Tashi said:
There are actually many different types of limits that are defined for sequences of functions. I'd have to look these up and re-study them before I could swear that none of them make the book's statement true.

A "sequence of functions" would require the domains to be the same though...
The only option I could think is define the sequence of sequences
h = [g(t_1), \ldots, g(t_N), 0, 0, \ldots]
and then look at limits. Err.

The book really should have used step functions.
 
pwsnafu said:
A "sequence of functions" would require the domains to be the same though...
.

Yes, by many definitions it would. But statisticians say things like "For large N the binomial distribution can be approximated by a normal distribution". That's a statement connecting a function with a discrete domain to a function with a continuous domain. I don't know if people bother to formulate that type of approximation as a theorem about a limit of a sequence of functions, but I think if they wanted to, they could come up with something. There are so many possibilities for making up definitions!
 
This a nice example of the difference between math and engineering.

In engneering, any "real world" signal will be bandwith limited, and the statement in the book is true for bandwidth-limited signals.

But I agree that the book's "explanation" isn't much more than an arm-waving reason why that seems plausible.
 

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