Limit of Sequence: Find w/o L'Hosp. Rule

In summary, the limit \lim_{n \rightarrow \infty}\frac{n}{\log_{10}{n}} can be solved using L'Hospital's rule or by finding easily manageable subsequences. There is also the option of using an epsilon-delta proof of L'Hospital's rule.
  • #1
dobry_den
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Homework Statement



Find the following limit:

[tex]\lim_{n \rightarrow \infty}\frac{n}{\log_{10}{n}}[/tex]

The Attempt at a Solution



It's easy to find the limit using L'Hospital rule (after having used Heine theorem to transform the sequence into a function):

[tex]\lim_{x \rightarrow \infty}\frac{x}{\log_{10}{x}} = \lim_{x \rightarrow \infty}\frac{1}{\frac{1}{x\log{10}}} = +\infty[/tex]

Is there any way of solving it without L'Hospital rule?

If I was to use the definition, then for every K, there should be such n_0 that for every n>n_0, (n/log_10(n)) > K. But I don't know how to solve this inequality. Any help would be greatly appreciated, thanks in advance!
 
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  • #2
There are easily manageable subsequences like:
[tex]\frac{10^{10^k}}{\log_{10}10^{10^k}}=10^{10^k-k}[/tex]
which clearly go to infinitely.

You could also work through an epsilon-delta proof of l'Hospital's rule.
 
  • #3
The subsequence way of solving is elegant, i didn't realize it... thanks a lot!
 

1. What is a limit of sequence?

A limit of sequence is the value that a sequence approaches as its terms get closer and closer, but never actually reaches.

2. How do I find the limit of a sequence without using L'Hospital's rule?

To find the limit of a sequence without using L'Hospital's rule, you can use various techniques such as algebraic manipulation, factoring, or applying limit laws. You can also use the squeeze theorem or direct substitution for specific types of sequences.

3. What is L'Hospital's rule and why is it not always applicable for finding limits of sequences?

L'Hospital's rule is a mathematical theorem that helps evaluate limits of functions in certain indeterminate forms. It is not always applicable for finding limits of sequences because sequences are not necessarily functions, and L'Hospital's rule requires that the limit of the numerator and denominator exist, which may not always be the case for sequences.

4. Can I use L'Hospital's rule to find the limit of any sequence?

No, L'Hospital's rule can only be applied to certain types of sequences, such as ratios of functions or exponential and logarithmic sequences. It cannot be used for all types of sequences.

5. Are there any other methods for finding the limit of a sequence besides L'Hospital's rule?

Yes, there are several other methods for finding the limit of a sequence, such as using limit laws, the squeeze theorem, or direct substitution. You can also use algebraic manipulation or factoring to simplify the sequence and then find the limit.

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