Determining the convergence or divergence of a sequence using direct comparison

Click For Summary
SUMMARY

The discussion focuses on determining the convergence or divergence of the series $$\sum_{n = 2}^{\infty} \frac{{(\ln(n))^{12}}{n^{\frac{9}{8}}}$$ using direct comparison with the series $$\sum_{n = 1}^{\infty} \frac{1}{{n}^{\frac{1}{8}}}$$. The user aims to prove that $$\frac{{(\ln(n))^{12}}{n^{\frac{9}{8}}} \ge \frac{1}{{n}^{\frac{1}{8}}}$$ to establish divergence. The guiding principle is that $\ln n$ grows slower than any positive power of $n$, indicating that the series behaves similarly to $$\sum \frac{1}{n^{9/8}}$$, which converges.

PREREQUISITES
  • Understanding of series convergence tests, specifically the Direct Comparison Test.
  • Familiarity with logarithmic functions and their growth rates.
  • Knowledge of p-series and their convergence criteria.
  • Basic calculus concepts, particularly limits and asymptotic behavior.
NEXT STEPS
  • Study the Direct Comparison Test in detail to understand its application in series convergence.
  • Learn about the behavior of logarithmic functions compared to polynomial functions.
  • Explore p-series convergence criteria and how they apply to different values of p.
  • Investigate asymptotic analysis to better understand the growth rates of functions.
USEFUL FOR

Mathematicians, students studying calculus or real analysis, and anyone interested in series convergence and comparison techniques.

tmt1
Messages
230
Reaction score
0
I have

$$\sum_{n = 2}^{\infty} \frac{{(\ln\left({n}\right)})^{12}}{n^{\frac{9}{8}}}$$

We can compare it to $ \frac{1}{{n}^{\frac{1}{8}}}$. $ \sum_{n = 1}^{\infty} \frac{1}{{n}^{\frac{1}{8}}}$ diverges because $p < 1$ in this case. So, if I can prove that $ \frac{{(\ln\left({n}\right)})^{12}}{n^{\frac{9}{8}}} \ge \frac{1}{{n}^{\frac{1}{8}}}$ then that would mean $\sum_{n = 2}^{\infty} \frac{{\ln\left({n}\right)}^{12}}{n^{\frac{9}{8}}}$ diverges. Or $ \frac{{(\ln\left({n}\right)})^{12}}{n^{\frac{9}{8}}} - \frac{1}{{n}^{\frac{1}{8}}} \ge 0 $.

How can I prove this?
 
Physics news on Phys.org
tmt said:
I have

$$\sum_{n = 2}^{\infty} \frac{{(\ln\left({n}\right)})^{12}}{n^{\frac{9}{8}}}$$

We can compare it to $ \frac{1}{{n}^{\frac{1}{8}}}$. $ \sum_{n = 1}^{\infty} \frac{1}{{n}^{\frac{1}{8}}}$ diverges because $p < 1$ in this case. So, if I can prove that $ \frac{{(\ln\left({n}\right)})^{12}}{n^{\frac{9}{8}}} \ge \frac{1}{{n}^{\frac{1}{8}}}$ then that would mean $\sum_{n = 2}^{\infty} \frac{{\ln\left({n}\right)}^{12}}{n^{\frac{9}{8}}}$ diverges. Or $ \frac{{(\ln\left({n}\right)})^{12}}{n^{\frac{9}{8}}} - \frac{1}{{n}^{\frac{1}{8}}} \ge 0 $.

How can I prove this?
The guiding principle is that $\ln n$ increases more slowly than any (positive) power of $n$. So for convergence purposes $\sum\frac{(\ln n)^{12}}{n^{9/8}}$ behaves pretty much like $\sum\frac1{n^{9/8}}$. Can you take it from there?
 

Similar threads

  • · Replies 3 ·
Replies
3
Views
4K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 6 ·
Replies
6
Views
4K
  • · Replies 15 ·
Replies
15
Views
3K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 17 ·
Replies
17
Views
5K
Replies
3
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K