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Ch. 11 - Power Series
Briggs - Calculus: Early Transcendentals 3rd Edition
Briggs3rd EditionCalculus: Early TranscendentalsISBN: 9780136847243Non è quello che usi tu?Cambia libro di testo
Capitolo 11, Problema 11.3.67e

Explain why or why not Determine whether the following statements are true and give an explanation or counterexample.
e. The Taylor series for an even function centered at 0 has only even powers of x.

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Recall the definition of an even function: a function \( f(x) \) is even if \( f(-x) = f(x) \) for all \( x \) in its domain.
Consider the Taylor series of \( f(x) \) centered at 0, which is given by \( f(x) = \sum_{n=0}^{\infty} \frac{f^{(n)}(0)}{n!} x^n \).
Substitute \( -x \) into the Taylor series: \( f(-x) = \sum_{n=0}^{\infty} \frac{f^{(n)}(0)}{n!} (-x)^n = \sum_{n=0}^{\infty} \frac{f^{(n)}(0)}{n!} (-1)^n x^n \).
Since \( f \) is even, \( f(-x) = f(x) \), so the series must satisfy \( \sum_{n=0}^{\infty} \frac{f^{(n)}(0)}{n!} (-1)^n x^n = \sum_{n=0}^{\infty} \frac{f^{(n)}(0)}{n!} x^n \). This implies that terms with odd powers \( n \) must have zero coefficients because \( (-1)^n = -1 \) for odd \( n \), which would otherwise change the sign.
Therefore, the Taylor series for an even function centered at 0 contains only even powers of \( x \), confirming the statement is true.

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