Does bn Converge to Zero if an Diverges and an*bn Converges?

In summary, if sequence a_n diverges to infinity and sequence a_n * b_n converges, then it can be proven that sequence b_n must converge to zero by using the given information about the convergence of a_n * b_n and the divergence of a_n.
  • #1
sitia
25
0

Homework Statement



If sequence an diverges to infinity and sequence an*bn converges then how do I prove that sequence bn must converge to zero?


Homework Equations





The Attempt at a Solution


I really don't know how to go about this so any help would be so appreciated.
Thanks
 
Physics news on Phys.org
  • #2
If [itex]a_nb_n[/itex] converges, to, say L, then, given any [itex]\epsilon> 0[/itex], there exist [itex]N_1[/itex] such that if [itex]n> N_1[/itex], [itex]|a_nb_n- L|< \epsilon[/itex].

Since [itex]a_n[/itex] diverges to infinity, then, given any X>0, there exist [itex]N_2[/itex] such that if [itex]n> N_2[/itex], [itex]a_n> X[/itex].

Take n greater than the larger of [itex]N_1[/itex] and [itex]N_2[/itex] and use both of those.
 

FAQ: Does bn Converge to Zero if an Diverges and an*bn Converges?

1. What is "Proving Convergence to 0"?

"Proving Convergence to 0" is a mathematical concept that refers to showing that a sequence of numbers approaches 0 as the number of terms in the sequence increases. It is often used in calculus and analysis to determine the behavior of a series or sequence.

2. How do you prove convergence to 0?

To prove convergence to 0, you must show that for any small number ε (epsilon), there exists a corresponding number N such that the absolute value of the difference between the terms of the sequence and 0 is less than ε for all values of n greater than or equal to N. This is known as the ε-N definition of convergence.

3. Why is proving convergence to 0 important?

Proving convergence to 0 allows us to determine the behavior of a sequence or series and make predictions about its limit. It is also an important tool in calculus for evaluating integrals and derivatives, as well as in the study of infinite series.

4. What are some common methods for proving convergence to 0?

Some common methods for proving convergence to 0 include using the ε-N definition, the comparison test, the ratio test, and the root test. Each method has its own conditions and limitations, so it is important to choose the appropriate method for the given sequence.

5. Can a sequence converge to 0 without being strictly decreasing?

Yes, a sequence can converge to 0 without being strictly decreasing. The sequence must still satisfy the ε-N definition of convergence, but it is possible for the terms to fluctuate or even increase as long as they eventually approach 0. This is known as convergence in the mean or Cesàro convergence.

Similar threads

Replies
2
Views
939
Replies
2
Views
654
Replies
2
Views
1K
Replies
4
Views
1K
Replies
6
Views
1K
Replies
4
Views
4K
Replies
5
Views
1K
Back
Top