How can the lim inf and lim sup be used to solve this mystery?

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SUMMARY

The discussion centers on the mathematical inequality involving the limit inferior (lim inf) of sequences, specifically demonstrating that lim inf (an × bn) is greater than or equal to lim inf an × lim inf bn. The user outlines their initial approach, referencing fixed natural numbers and the properties of infimums. They express confusion regarding the transition to the supremum of the infimums, which is clarified through the relationship between lim inf and sup inf. Ultimately, the user resolves their query independently, indicating a successful understanding of the concept.

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  • Understanding of limit inferior (lim inf) and limit superior (lim sup) in real analysis
  • Familiarity with sequences and their properties, particularly infimum and supremum
  • Basic knowledge of mathematical inequalities and their proofs
  • Experience with mathematical notation and terminology used in analysis
NEXT STEPS
  • Study the properties of lim inf and lim sup in greater depth
  • Explore examples of sequences that illustrate the application of lim inf and lim sup
  • Learn about convergence and divergence of sequences in real analysis
  • Investigate the implications of these concepts in functional analysis and topology
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Students of mathematics, particularly those studying real analysis, educators teaching limit concepts, and anyone seeking to deepen their understanding of sequence behavior in mathematical contexts.

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Homework Statement


Demonstrate lim inf an × lim inf bn ≤ lim inf (an × bn)

Homework Equations

The Attempt at a Solution


Fixxed n € N , n≤ k , inf ak ≤ ak , inf bk≤ bk =》inf ak × inf bk ≤ (ak × bk) . Being inf ak × inf bk ≤ any element from {ak} × {bk} it`s logical that inf ( ak × bk ) ≥ inf ak × inf bk... the part i don't get it's the following: the teacher suddently writes : sup inf ( ak × bk ) ≥ sup inf ak × sup inf bk.. the part after this is much easier since sup inf ak = lim inf an , sup inf bk = lim inf bk and sup inf ( ak × bk ) = lim inf ( ak×bk)
Can anyone solve this mistery? Many Thanks in advance
 
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