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Determine the convergence of the given series

Consider the series

    \[ \sum_{n=1}^{\infty} (-1)^n \left( \frac{1 \cdot 3 \cdot 5 \cdots (2n-1)}{2 \cdot 4 \cdot 6 \cdots (2n)} \right)^3. \]

Determine whether the series converges or diverges. If it converges, determine whether it converges conditionally or absolutely.


The given series is absolutely convergent.

Proof. We can see this since

    \[ \sum_{n=1}^{\infty} \left| (-1)^n \left(\frac{1 \cdot 3 \cdot 5 \cdots (2n-1)}{2 \cdot 4 \cdot 6 \cdots (2n)}\right)^3 \right| = \sum_{n=1}^{\infty} \left( \frac{1 \cdot 3 \cdot 5 \cdots (2n-1)}{2 \cdot 4 \cdot 6 \cdots (2n)} \right)^3. \]

This series converges by a previous exercise (Section 10.16, Exercise #18) with k = 3. \qquad \blacksquare

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