How can I prove this from Roger Penrose's Road to Reality?

Click For Summary

Discussion Overview

The discussion revolves around proving properties of the function h(x) defined piecewise, particularly its classification as a C∞ function. Participants explore methods to demonstrate the continuity and differentiability of h(x) across its domain.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant seeks assistance in proving properties of the function h(x), specifically its classification as C∞.
  • Another participant points out that the initial post lacks a clear question regarding what needs to be proved.
  • A third participant notes that the definition provided is incomplete for proving properties and suggests proving continuity through limits.
  • The original poster clarifies their intent to prove that h(x) has an infinite number of derivatives and that the slope is zero at the origin.
  • One participant proposes proving that h is C^1 and then using induction to establish that it is C^n for all n ≥ 1.

Areas of Agreement / Disagreement

Participants express differing views on how to approach the proof, with some focusing on continuity and others on differentiability. There is no consensus on a single method or approach to proving the properties of h(x).

Contextual Notes

The discussion highlights the need for a complete definition of h(x) and the assumptions regarding its differentiability. The approach to proving its properties may depend on the chosen method, such as limits or Taylor series.

Who May Find This Useful

This discussion may be useful for students or individuals interested in real analysis, particularly those studying properties of piecewise functions and differentiability.

Etienne
Messages
24
Reaction score
0
Hi, I've been reading Roger Penrose's Road to Reality during my free time, and I am trying to do all the proofs I possibly can, although I am quickly reaching my limit.

Would somebody help me prove this?

h(x)=0 if x ≤ 0
h(x) = e-1/x if x > 0

Thank you in advance :)
 
Mathematics news on Phys.org
Hi,

Read what you wrote and then maybe understand that there is no question in your post. Prove what ?
 
This is an incomplete definition of h(x). You cannot prove a definition.
You can prove that h is continuous by looking at a specific limit.
 
I'm so sorry, I see I only wrote the definition of the function.

It is said that the function h(x) is of class C in the domain. What I understand is that it must have an infinite number of derivatives, and that at the slope must be 0 at the origin.

However, I haven't been able to prove this and apparently neither could a friend. It is probably much easier than I think, but I haven't been able to do it.

Could I approach this with a Taylor series? Logarithms? I apologize for the anterior and thanks again.
 
Prove that it is ##C^1## and then use induction to prove that it is ##C^n## for all ##n\geq 1##.
 
  • Like
Likes   Reactions: Etienne and BvU

Similar threads

  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 36 ·
2
Replies
36
Views
6K
  • · Replies 2 ·
Replies
2
Views
13K
  • · Replies 3 ·
Replies
3
Views
6K
  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 11 ·
Replies
11
Views
10K
  • · Replies 9 ·
Replies
9
Views
4K
  • · Replies 5 ·
Replies
5
Views
2K
Replies
3
Views
2K
  • · Replies 9 ·
Replies
9
Views
3K