Why the triangle inequality is greater than the 2 max{f(x),g(x)}

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SUMMARY

The discussion centers on the proof from Sheldon Axler's book on Measure Theory, specifically regarding the properties of the vector space ##L^p(\mu)## over ##\mathbb{R}##. The key assertion is that if the inequality ##\|f+g\|_{p}^p\leq 2^p(\|f\|_{p}^p+\|g\|_{p}^{p})## holds, along with nonzero homogeneity, then ##L^p(\mu)## qualifies as a vector space with standard addition and scalar multiplication. The proof utilizes the triangle inequality and the properties of maximum values to establish the relationship between the norms of the functions involved.

PREREQUISITES
  • Understanding of ##L^p## spaces in functional analysis
  • Familiarity with the triangle inequality in normed spaces
  • Knowledge of scalar multiplication and vector space properties
  • Basic concepts of measure theory as presented in Axler's book
NEXT STEPS
  • Study the properties of ##L^p(\mu)## spaces in detail
  • Learn about the triangle inequality and its applications in functional analysis
  • Explore the concept of nonzero homogeneity in vector spaces
  • Review examples of functions in ##L^p(\mu)## to solidify understanding of the norms
USEFUL FOR

Mathematicians, students of functional analysis, and anyone studying measure theory who seeks to deepen their understanding of vector spaces and their properties.

cbarker1
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TL;DR
I am working on proving the L^p of measureable space (X,S,u) is a vector space. I am lost on why the the triangle intequalty is greater than the 2 max{f(x),g(x)} for a fix x in X.
I am reading Sheldon's Axler Book on Measure theory. He is proving that ##L^p(\mu)## is a Vector space over ##\mathbb{R}.## He claims that if ##\|f+g\|_{p}^p\leq 2^p(\|f\|_{p}^p+\|g\|_{p}^{p})## and nonzero homogenity holds true, then ##L^p{\mu}## is true with the standard addition and scalar multiplication. He starts with the following assumption:

Suppose that ##f,g\in L^p(\mu)## are arbitrary. Then if ##x\in X## is an arbitrary fix element of ##X,## then
##\begin{align*} |f(x)+g(x)|^p&\leq_{\text{triangle inequality}} (|f(x)|+|g(x)|)^p\\ &\leq_{\text{why?}} (2\max{|f(x)|,|g(x)|})^p\\ &\leq2^p(|f(x)|^p+|g(x)|^p)\end{align*}##

If you can explain whys in this proof then I will be able to understand the proof.

Thanks,

Carter Barker
 
Last edited:
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Because if, e.g., |f|>=|g|, then |f|+|g|<=|f|+|f|=2|f|
 

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