Mathematics & Numerical Analysis

Step-by-step analysis of each option for $a_n = \frac{1}{\sqrt{n}} = n^{-1/2}$:

• Incorrect: By the Alternating Series (Leibniz) Test, since $a_n \gt 0$, $a_n$ is monotonically decreasing to $0$, $\sum (-1)^n a_n$ converges (conditionally).
• Incorrect: $\sum (a_n)^2 = \sum \frac{1}{n}$, which is the divergent harmonic series ($p = 1$).
• Correct: $\sum_{n=1}^\infty (a_n)^3 = \sum_{n=1}^\infty \left(\frac{1}{\sqrt{n}}\right)^3 = \sum_{n=1}^\infty \frac{1}{n^{3/2}}$. This is a standard $p$-series with $p = 3/2 \gt 1$, which is convergent.
• Incorrect: $\sum a_n = \sum \frac{1}{n^{1/2}}$ is a $p$-series with $p = 1/2 \le 1$, which is divergent.

Analysis of convergence theorems in real analysis:

• Correct: A fundamental theorem states that if a series is absolutely convergent (i.e., $\sum |a_n|$ converges), then the series $\sum a_n$ itself is necessarily convergent.
• Incorrect: The converse is false; for example, $\sum \frac{(-1)^n}{n}$ converges but is not absolutely convergent.
• Incorrect: Conditional convergence means the series converges but does NOT converge absolutely, which is the exact opposite.
• Incorrect: Conditionally convergent series can never be absolutely convergent by definition.

Step-by-step telescoping summation:
Each term can be decomposed into partial fractions:

$$\frac{1}{k(k+1)} = \frac{1}{k} - \frac{1}{k+1}$$

Writing out the sum for $k = 1$ to $9$:

$$S = \left(1 - \frac{1}{2}\right) + \left(\frac{1}{2} - \frac{1}{3}\right) + \left(\frac{1}{3} - \frac{1}{4}\right) + \dots + \left(\frac{1}{9} - \frac{1}{10}\right)$$

All intermediate terms cancel out:

$$S = 1 - \frac{1}{10} = \frac{9}{10}$$

• Incorrect: $10/9$ is the reciprocal.
• Correct: Accurately equals $9/10$.
• Incorrect: $5/6$ would be the sum up to 5 terms.
• Incorrect: $6/5$ is invalid.