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\( \sum _ { n = 0 } ^ { \infty } n ! ( 2 x - 1 ) ^ { n } \)

Ask by Cox Peters. in the Philippines
Mar 10,2025

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The series \( \sum_{n=0}^{\infty} n! (2x-1)^n \) converges only when \( x = \frac{1}{2} \), and its sum is 1 at that point.

Solution

1. We start with the series \[ \sum_{n=0}^{\infty} n! (2x-1)^n. \] Introducing the substitution \( z = 2x-1 \), the series becomes \[ \sum_{n=0}^{\infty} n! \, z^n. \] 2. We apply the ratio test. For the general term \[ a_n = n! \, z^n, \] the ratio of consecutive terms is \[ \left|\frac{a_{n+1}}{a_n}\right| = \left|\frac{(n+1)! \, z^{n+1}}{n! \, z^n}\right| = |(n+1)z| = (n+1)|z|. \] 3. We take the limit as \( n \to \infty \): \[ \lim_{n \to \infty} (n+1)|z| = \infty \quad \text{if} \quad |z| > 0. \] Hence, the ratio test shows divergence for any \( z \) with \( |z| > 0 \). 4. The series converges only when \[ |z| = 0, \] which implies \[ 2x-1 = 0. \] Solving for \( x \) gives \[ x = \frac{1}{2}. \] 5. When \( x = \frac{1}{2} \), we have \( z = 0 \). Then the series becomes \[ \sum_{n=0}^{\infty} n! \, 0^n. \] Note that \( 0^0 \) is defined as \( 1 \) in the context of power series, and for \( n \geq 1 \), \( 0^n = 0 \). Therefore, the series reduces to \[ 1 + 0 + 0 + \cdots = 1. \] 6. Conclusion: - The series converges only for \( x = \frac{1}{2} \). - At \( x = \frac{1}{2} \), the sum of the series is \( 1 \).

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To evaluate the series \( \sum_{n=0}^{\infty} n! (2x - 1)^n \), we can recognize that the series resembles a known function. Specifically, we can utilize the exponential generating function for the factorial. The series can be rewritten as follows: \[ \sum_{n=0}^{\infty} n! y^n = \frac{1}{1 - y} \] for \( |y| < 1 \). In our case, let \( y = 2x - 1 \). Therefore, the series converges for \( |2x - 1| < 1 \), or \( 0 < x < 1 \). So we have: \[ \sum_{n=0}^{\infty} n! (2x - 1)^n = e^{(2x - 1)} \] However, this is just a transformation, and we need to handle the sum properly: To find the function corresponding to the series, if we take the sum \( \sum_{n=0}^{\infty} \frac{(2x - 1)^n}{n!} \), it yields \( e^{2x-1} \), indicating that our original series diverges for most values. To find the proper sum expansion, we'd investigate methods like using integral transforms or addressing convergence regions. While we present this, proper techniques like power series and complex analysis might give cleaner conclusions about evaluating convergence under real conditions, leading us to realize higher methodologies can yield cleaner results. In any case, the series diverges for most values, leading to fascinating behavior under constraints and functional analysis in complex variables!

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