Product of 2 Increasing Sequences Not Necessarily Increasing

In summary, the conversation discusses the concept of eventually increasing sequences and gives an example to show that the product of two eventually increasing sequences is not necessarily eventually increasing. The conversation also includes a hint to consider using a negative sequence as a counterexample.
  • #1
Easy_as_Pi
31
0

Homework Statement


Give an example to show that it is not necessarily true that the product of two eventually increasing sequences is eventually increasing.


Homework Equations


a sequence is eventually increasing if for N[itex]\in[/itex] natural numbers, a[itex]_{n+1}[/itex] [itex]\geq[/itex]a[itex]_{n}[/itex] for all n>N.


The Attempt at a Solution


So, I know this is merely proof by counterexample. I find one example to show that the product of two eventually increasing sequences is not necessarily eventually increasing. The only catch is that I have no idea where to start. There are infinitely many eventually increasing sequences I could multiply together. I know the end goal is to show that a[itex]_{n+1}[/itex] - a[itex]_{n}[/itex] is decreasing or eventually decreasing for all n.
So, ideally, I'd end up with something like -x[itex]^{2}[/itex] after a[itex]_{n+1}[/itex] - a[itex]_{n}[/itex]. I don't want a specific example which will solve this problem. Some guidance as to where to begin would be greatly appreciated, though!
Thanks!
 
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  • #2
Try picking an eventually increasing sequence whose values are negative.
 
  • #3
Sorry that I took an eternity to reply. I was at a study session for linear algebra. Thanks so much, Dick. I think you've helped on every question I've posted here. I really appreciate it. I tripped myself up by only thinking about positive sequences. I took increasing and mistakenly correlated it with positive, too. Immediately after your hint, I thought about -1/n, and then found my solution. This series and sequences course has managed to confuse me more than any previous math class.
 

1. What is the definition of a "Product of 2 Increasing Sequences Not Necessarily Increasing"?

A product of 2 increasing sequences not necessarily increasing is a mathematical concept where two sequences of numbers are multiplied together, where each individual sequence is increasing, but the resulting product may not be increasing.

2. How do you determine if a product of 2 increasing sequences is increasing or not?

To determine if a product of 2 increasing sequences is increasing or not, you can analyze the individual sequences and their patterns. If both sequences are increasing, then the product will also be increasing. However, if one or both sequences are not strictly increasing, the product may not be increasing.

3. Can a product of 2 increasing sequences not necessarily increasing ever be decreasing?

Yes, a product of 2 increasing sequences not necessarily increasing can be decreasing. This is because even though the individual sequences are increasing, the product can be influenced by the values in both sequences and may result in a decrease.

4. What real-life applications does the concept of "Product of 2 Increasing Sequences Not Necessarily Increasing" have?

The concept of "Product of 2 Increasing Sequences Not Necessarily Increasing" can be applied in various fields such as economics, biology, and statistics. For example, in economics, it can be used to analyze the relationship between two variables and their impact on a product. In biology, it can be used to study the growth of two different organisms and how their combined growth affects the overall population. In statistics, it can be used to analyze the trend of two variables and their correlation.

5. What is the difference between "Product of 2 Increasing Sequences Not Necessarily Increasing" and "Product of 2 Increasing Sequences"?

The main difference between these two concepts is that in "Product of 2 Increasing Sequences", both sequences must strictly be increasing for the product to also be increasing, whereas in "Product of 2 Increasing Sequences Not Necessarily Increasing", one or both sequences can have non-increasing values and still result in an increasing or decreasing product.

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