Subjects calculus

Infinite Factorial Series A92354

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1. **Problem statement:** Find the value of the infinite series \(\sum_{k=1}^\infty \frac{1}{k!}\). 2. **Recall the exponential series:** The Maclaurin series expansion for \(e^x\) is given by $$e^x = \sum_{k=0}^\infty \frac{x^k}{k!} = 1 + \frac{x}{1!} + \frac{x^2}{2!} + \frac{x^3}{3!} + \cdots$$ 3. **Apply the formula for \(x=1\):** $$e^1 = e = \sum_{k=0}^\infty \frac{1^k}{k!} = \sum_{k=0}^\infty \frac{1}{k!} = 1 + \sum_{k=1}^\infty \frac{1}{k!}$$ 4. **Isolate the series:** $$\sum_{k=1}^\infty \frac{1}{k!} = e - 1$$ 5. **Interpretation:** The series sums all reciprocals of factorials starting from \(k=1\), which converges to \(e - 1\). **Final answer:** $$\sum_{k=1}^\infty \frac{1}{k!} = e - 1$$