We need to determine if the series $\sum_{n=1}^{\infty} \frac{3n+1}{n^3-4}$ converges or diverges.

AnalysisSeries ConvergenceLimit Comparison TestInfinite Series
2025/3/9

1. Problem Description

We need to determine if the series n=13n+1n34\sum_{n=1}^{\infty} \frac{3n+1}{n^3-4} converges or diverges.

2. Solution Steps

We can use the limit comparison test. Let an=3n+1n34a_n = \frac{3n+1}{n^3-4}. We will compare it to bn=1n2b_n = \frac{1}{n^2}. Note that n=11n2\sum_{n=1}^{\infty} \frac{1}{n^2} converges because it is a pp-series with p=2>1p=2 > 1.
We calculate the limit:
limnanbn=limn3n+1n341n2=limnn2(3n+1)n34=limn3n3+n2n34=limn3+1n14n3=3+010=3\lim_{n\to\infty} \frac{a_n}{b_n} = \lim_{n\to\infty} \frac{\frac{3n+1}{n^3-4}}{\frac{1}{n^2}} = \lim_{n\to\infty} \frac{n^2(3n+1)}{n^3-4} = \lim_{n\to\infty} \frac{3n^3+n^2}{n^3-4} = \lim_{n\to\infty} \frac{3+\frac{1}{n}}{1-\frac{4}{n^3}} = \frac{3+0}{1-0} = 3
Since the limit is a positive finite number (3), the series n=1an\sum_{n=1}^{\infty} a_n and n=1bn\sum_{n=1}^{\infty} b_n either both converge or both diverge. Since n=11n2\sum_{n=1}^{\infty} \frac{1}{n^2} converges, then n=13n+1n34\sum_{n=1}^{\infty} \frac{3n+1}{n^3-4} also converges. Note that for n=1n=1, the term is 43\frac{4}{-3}, for n=2n=2, the term is 74\frac{7}{4}.
So, the original series starts from n=3n=3 since n34>0n^3-4>0 when n>43n> \sqrt[3]{4}.
Since we only are concerned with convergence, the series converges even if some initial terms may be undefined.

3. Final Answer

The series converges.

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