Math Problem Statement

Show that the series ∑_{n ≥ 2} (-1)^n / [n(log n)^p] is absolutely convergent if p > 1.

Solution

The given series is:

n2(1)n1n(logn)p\sum_{n \geq 2} (-1)^n \frac{1}{n (\log n)^p}

We are tasked with showing that this series is absolutely convergent if p>1p > 1.

Step-by-step solution:

  1. Absolute Convergence: For absolute convergence, we need to check the convergence of the series of the absolute values of the terms:

    n2(1)n1n(logn)p=n21n(logn)p\sum_{n \geq 2} \left| (-1)^n \frac{1}{n (\log n)^p} \right| = \sum_{n \geq 2} \frac{1}{n (\log n)^p}

    This is now a non-alternating positive series.

  2. Convergence Test: To determine the convergence of this series, we will use the integral test. The integral test states that if the function f(n)=1n(logn)pf(n) = \frac{1}{n (\log n)^p} is positive, continuous, and decreasing for n2n \geq 2, then the convergence of the integral:

    21x(logx)pdx\int_2^\infty \frac{1}{x (\log x)^p} \, dx

    is equivalent to the convergence of the series n21n(logn)p\sum_{n \geq 2} \frac{1}{n (\log n)^p}.

  3. Integral Calculation: Let's calculate the improper integral:

    21x(logx)pdx\int_2^\infty \frac{1}{x (\log x)^p} \, dx

    To solve this, we use the substitution u=logxu = \log x, which gives du=1xdxdu = \frac{1}{x} dx. The limits of integration change accordingly:

    • When x=2x = 2, u=log2u = \log 2,
    • When x=x = \infty, u=u = \infty.

    Thus, the integral becomes:

    log21updu\int_{\log 2}^\infty \frac{1}{u^p} \, du

    The integral log21updu\int_{\log 2}^\infty \frac{1}{u^p} \, du converges if and only if p>1p > 1, because for p=1p = 1, it would diverge as a logarithmic integral, and for p<1p < 1, the exponent is too small for convergence.

  4. Conclusion: The series n21n(logn)p\sum_{n \geq 2} \frac{1}{n (\log n)^p} converges if p>1p > 1 based on the integral test. Hence, the original series is absolutely convergent for p>1p > 1.

Would you like further details on any part of the explanation?

Here are some related questions:

  1. How does the integral test work for series convergence?
  2. Why is the substitution u=logxu = \log x helpful in the integral calculation?
  3. What is the difference between absolute convergence and conditional convergence?
  4. What happens to the series if p=1p = 1?
  5. How would you handle this series if it didn't alternate (i.e., without the (1)n(-1)^n factor)?

Tip: The integral test is a powerful tool for series with logarithmic or power terms. Always check the conditions where the function behaves continuously and decreases to apply it.

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Math Problem Analysis

Mathematical Concepts

Series Convergence
Logarithmic Series
Integral Test

Formulas

Absolute convergence: ∑ |(-1)^n / [n(log n)^p]| = ∑ 1 / [n(log n)^p]
Integral for convergence: ∫ 1 / [x(log x)^p] dx

Theorems

Integral Test

Suitable Grade Level

Undergraduate Mathematics (Calculus II or III)