How Is Divergence to Infinity Defined in Contrast to General Divergence?

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Discussion Overview

The discussion focuses on the definitions of divergence in sequences, specifically contrasting the general definition of divergence with the specific case of divergence to infinity. Participants explore the implications of these definitions and the relationships between them.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant presents the general definition of divergence in sequences and questions how it relates to the definition of divergence to infinity.
  • Another participant asserts that divergence to infinity is a sub-case of general divergence, suggesting that if the criteria for divergence to infinity are met, the general criteria must also be satisfied, though this needs to be proven.
  • A later reply mentions that the reverse implication does not hold, using oscillating sequences as a counterexample.
  • Further exploration is proposed on how to demonstrate that the second criteria implies the first, with a participant suggesting a proof by contradiction involving boundedness.
  • Another participant provides a less formal argument relating the definitions by defining a specific epsilon based on the difference between L and M, indicating that if a_n exceeds M, it also satisfies the general divergence condition.

Areas of Agreement / Disagreement

Participants express differing views on the implications of the definitions, with some asserting that one definition implies the other while others provide counterexamples. The discussion remains unresolved regarding the proof of these implications.

Contextual Notes

The discussion includes complex quantifier manipulations and assumptions about boundedness that are not fully resolved. The implications of oscillating sequences as counterexamples are also noted but not conclusively addressed.

Mr Davis 97
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The general definition for a sequence to diverge is the negation of what it means for a sequence to converge: ##\forall L\in\mathbb{R}~\exists\epsilon>0~\forall N\in\mathbb{N}~\exists n\ge N##, ##|a_n - L| \ge \epsilon##. How does this general definition of divergence relate to the definition of a sequence diverging specifically to infinity, which is ##\forall M \in \mathbb{R} ~ \exists N \in \mathbb{N} \forall n \ge N##, ##a_n > M##?
 
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The second is a sub-case of the first, so if the second criteria is met, so will the first be. That needs to be proved. It is not difficult. The reverse implication does not hold. Oscillating sequences are a counterexample.
 
andrewkirk said:
The second is a sub-case of the first, so if the second criteria is met, so will the first be. That needs to be proved. It is not difficult. The reverse implication does not hold. Oscillating sequences are a counterexample.
How would I go about showing that the second criteria implies the first, out of curiosity? I tried to take a stab at it, but I got buried by the quantifiers...

EDIT: Actually, maybe I see how to do. If we argue by contradiction we see that ##a_n## must converge, and so is bounded. But that contradicts the fact that it is unbounded...
 
Not quite formally: Let L be any real number and M any number M > L. Define ##M - L = \epsilon##. Then eventually all ##a_n > M## which certainly meets the condition ##|a_n - L| \geq \epsilon##
 

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