How do i prove that the following series diverges?

  • Thread starter Thread starter Dell
  • Start date Start date
  • Tags Tags
    Series
Click For Summary
SUMMARY

The series defined as 1/ln((n)!) for n ranging from 2 to infinity diverges. The user initially attempted to apply the ratio test using the limit of An+1/An but found it inconclusive. By leveraging the inequality ln(n!) ≤ n ln(n), it is established that the series S = ∑ (1/ln(n!)) is greater than or equal to the series ∑ (1/n ln n), which is known to diverge. Therefore, the original series also diverges.

PREREQUISITES
  • Understanding of series convergence and divergence
  • Familiarity with logarithmic properties and inequalities
  • Knowledge of the ratio test for series
  • Basic concepts of factorial growth and its logarithmic behavior
NEXT STEPS
  • Study the properties of logarithms in relation to factorials
  • Learn about the integral test for series convergence
  • Explore the comparison test for series divergence
  • Investigate the asymptotic behavior of ln(n!) using Stirling's approximation
USEFUL FOR

Mathematicians, students studying calculus or real analysis, and anyone interested in series convergence and divergence proofs.

Dell
Messages
555
Reaction score
0
1/ln((n)!) [n goes from 2-infinity]

1/ln((2)!) + 1/ln((3)!) +1 /ln((4)!) +...+ 1/ln((n)!)

the first thing i thought of was de lambert

lim An+1/An
n->inf

=ln(x!)/ln((x+1)!)
=ln(x!)/ln(x!*(x+1))
=ln(x!)/[ln(x!)+ln(x+1)]

=1-ln(x+1)/[ln(x!)+ln(x+1)]=1
which doesn't tell me anything about the convergence /divegence of the series.

have i done something wrong somewhere here, or is there another way to solve this?
 
Physics news on Phys.org
I don't know if this is the most efficient way, but if you note that
ln(n!) = ln(n) + ln(n - 1) + ... <= n ln(n)
you can show that
S = \sum_{n = 2}^\infty \frac{1}{\ln(n!)} \ge \sum_{n = 2}^\infty \frac{1}{n \ln n}
 

Similar threads

  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 13 ·
Replies
13
Views
2K
Replies
8
Views
2K
  • · Replies 3 ·
Replies
3
Views
7K
  • · Replies 4 ·
Replies
4
Views
1K
  • · Replies 22 ·
Replies
22
Views
4K
Replies
2
Views
1K
  • · Replies 6 ·
Replies
6
Views
2K
Replies
4
Views
2K
  • · Replies 2 ·
Replies
2
Views
1K