Metric Space Diameters of Sets: Find Condition

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SUMMARY

The discussion focuses on identifying conditions in a metric space (X, d) that guarantee the existence of subsets A and B, where A is a subset of B, and the diameters of both sets are equal (diam(A) = diam(B)). The diameter is defined as diam(A) = sup{d(r, s): r, s ∈ A}. An example provided illustrates this concept, using sets A = (-∞, 5] ∪ [-5, ∞) and B = (-∞, 4] ∪ [-4, ∞). The key inquiry is to determine the specific conditions under which this equality holds.

PREREQUISITES
  • Understanding of metric spaces and their properties
  • Familiarity with the concept of diameter in metric spaces
  • Knowledge of supremum and infimum in real analysis
  • Basic set theory, particularly subset relations
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  • Research conditions for equality of diameters in metric spaces
  • Explore examples of metric spaces where diam(A) = diam(B)
  • Study the implications of subset relations on diameters in metric spaces
  • Investigate the role of compactness in metric spaces and its effect on diameters
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Mathematics students, particularly those studying real analysis or topology, as well as educators seeking to clarify concepts related to metric spaces and their properties.

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Homework Statement


Find a condition on a metric space (X,d) that ensures that there exist subsets A,B of X with A\subset B such that diam(A)=diam(B).

Homework Equations


diam(A)=\sup\{d(r,s):r,s\in A\};
A\subseteq B\implies diam(A)\leq diam(B).

The Attempt at a Solution


Well I know examples of where this is true (ie, let A=(-\infty,5]\cup [-5,\infty)\subset (-\infty,4]\cup [-4,\infty)=B). But I don't know which condition allows this to be true. Any help is good. Thank you!
 
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