How Does Sobolev Space Boundedness Relate to Different Norms in $R^n$?

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SUMMARY

The discussion focuses on the relationship between Sobolev space boundedness and different norms in \( \mathbb{R}^n \). It establishes that if \( r < s < t \), then for any \( \epsilon > 0 \), there exists a constant \( C > 0 \) such that \( \|f\|_{(s)} \leq \epsilon \|f\|_{(t)} + C \|f\|_{(r)} \) for all \( f \in H_t \). The importance of the domain of \( f \) is highlighted, particularly noting that \( f \) is a tempered distribution and its integration context (e.g., on the torus versus \( \mathbb{R}^n \)) significantly affects the analysis.

PREREQUISITES
  • Understanding of Sobolev spaces, specifically \( H^t \) spaces.
  • Familiarity with norms in functional analysis.
  • Knowledge of tempered distributions and their properties.
  • Basic concepts of integration in different domains, particularly \( \mathbb{R}^n \) and the torus.
NEXT STEPS
  • Study the properties of Sobolev spaces, focusing on the implications of boundedness in different norms.
  • Explore the concept of tempered distributions and their applications in analysis.
  • Investigate the differences in integration techniques on \( \mathbb{R}^n \) versus other domains like the torus.
  • Learn about the implications of the inequality \( \|f\|_{(s)} \leq \epsilon \|f\|_{(t)} + C \|f\|_{(r)} \) in various contexts.
USEFUL FOR

Mathematicians, particularly those specializing in functional analysis, Sobolev spaces, and distribution theory, will benefit from this discussion.

Danny2
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If $ r<s<t $ then for any $ ϵ>0 $there exists $ C>0 $ such that $ ∥f∥_{(s)}≤ϵ∥f∥_{(t)}+C∥f∥_{(r)} $for all $f∈H_t $

Can you please tell me how to start thinking of this problem? I really feel stuck and don't know where to start!
 
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Welcome, Danny! (Wave)

Some information is missing in the problem statement. What is the domain of $f$?
 
$f$ is a tempered distribution
 
What I mean is this: $f\in H^t(X)$ for some space $X$ -- what is $X$? It makes a difference, since, e.g., integration on the torus is different from integration on $\Bbb R^n$.
 
$R^n$
 

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