Proving the divergent integral of 1/f(x) as x-> infinity

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Homework Help Overview

The discussion revolves around proving that a function f(x), which is greater than or equal to x for all x, must be a linear polynomial if the integral of 1/f(x) diverges as x approaches infinity. The context involves understanding the behavior of functions and their integrals in relation to divergence.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants explore the validity of the original statement regarding the nature of f(x) and its implications for the divergence of the integral. Some question whether non-polynomial functions could satisfy the conditions, while others suggest specific forms of f(x) that might meet the criteria.

Discussion Status

The discussion is active, with participants presenting differing viewpoints on the nature of f(x). Some participants have offered examples and counterexamples, indicating a productive exploration of the topic without reaching a consensus.

Contextual Notes

There is an ongoing examination of the assumptions regarding the polynomial nature of f(x) and its relationship to the divergence of the integral. The requirement that f(x) be greater than or equal to x is a key constraint under discussion.

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Homework Statement


There exists a function f(x) such that the indefinite integral of 1/f(x) as x-> infinity diverges, and f(x) >= x for all values of x. Prove this function must be a linear polynomial.

Homework Equations


None that I know of.

The Attempt at a Solution


No idea where to start.
 
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You can't, as stated, it is not true. There exist many non-polynomial functions that are "asymptotic" to x such that the integral of 1/f(x) diverges. However, if you require that f(x) be a polynomial, then it is true that f must be linear.
 
HallsofIvy said:
You can't, as stated, it is not true. There exist many non-polynomial functions that are "asymptotic" to x such that the integral of 1/f(x) diverges. However, if you require that f(x) be a polynomial, then it is true that f must be linear.

f(x) must always be larger than x, are there any asymptotic functions that fit that criteria?
 
000 said:
f(x) must always be larger than x, are there any asymptotic functions that fit that criteria?

How about f(x)=x+1/x?
 

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