Math Problem Statement

Calcular

Calcular la convergencia o divergencia de la serie infinita \( \sum_{n=1}^\infty (-1)^{n-1} \frac{\ln(\ln(n))}{\sqrt{n}} \).

Solution

The problem involves the infinite series:

n=1(1)n1ln(ln(n))n\sum_{n=1}^\infty (-1)^{n-1} \frac{\ln(\ln(n))}{\sqrt{n}}

We will analyze whether this series converges or diverges and, if it converges, determine its behavior.


Step 1: Convergence of Alternating Series (Leibniz Test)

For an alternating series of the form n=1(1)nan\sum_{n=1}^\infty (-1)^n a_n, the Leibniz test states that the series converges if:

  1. ana_n is positive, decreasing, and
  2. limnan=0\lim_{n \to \infty} a_n = 0.

Here, an=ln(ln(n))na_n = \frac{\ln(\ln(n))}{\sqrt{n}}.


Step 2: Check the terms ana_n

  1. Positivity:

    • For n>1n > 1, both ln(n)>0\ln(n) > 0 and ln(ln(n))>0\ln(\ln(n)) > 0, so an>0a_n > 0.
  2. Limit as nn \to \infty:

    • Compute limnln(ln(n))n\lim_{n \to \infty} \frac{\ln(\ln(n))}{\sqrt{n}}.
    • The square root n\sqrt{n} grows much faster than ln(ln(n))\ln(\ln(n)), so: limnln(ln(n))n=0.\lim_{n \to \infty} \frac{\ln(\ln(n))}{\sqrt{n}} = 0.
  3. Decreasing:

    • To check if ana_n is decreasing, observe the derivative of f(n)=ln(ln(n))nf(n) = \frac{\ln(\ln(n))}{\sqrt{n}}. The numerator ln(ln(n))\ln(\ln(n)) grows slower than the denominator n\sqrt{n}, so ana_n decreases for large nn.

Thus, ana_n satisfies all conditions of the Leibniz test.


Step 3: Conclusion

By the Leibniz test, the series converges.


Would you like a more detailed analysis or help with evaluating the series further? Here are some related questions to explore:

  1. What is the exact convergence criterion for alternating series?
  2. How does the growth rate of ln(ln(n))\ln(\ln(n)) compare to other logarithmic terms?
  3. What happens if we remove the alternating factor (1)n1(-1)^{n-1}?
  4. How do other convergence tests (e.g., ratio test, root test) apply here?
  5. What is the physical or practical interpretation of such alternating series?

Tip: For alternating series, always verify positivity, limit behavior, and monotonicity of the terms.

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Math Problem Analysis

Mathematical Concepts

Infinite Series
Alternating Series
Convergence Tests

Formulas

General term of the series \( a_n = \frac{\ln(\ln(n))}{\sqrt{n}} \)
Leibniz Test for alternating series

Theorems

Leibniz Test for Alternating Series

Suitable Grade Level

Grades 11-12, Undergraduate Mathematics