Power Series Expansion for Fresnel Cosine Integral Function
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Theorem
- $\ds \map {\operatorname C} x = \sqrt {\frac 2 \pi} \sum_{n \mathop = 0}^\infty \paren {-1}^n \frac {x^{4 n + 1} } {\paren {4 n + 1} \paren {2 n}!}$
where $\operatorname C$ denotes the Fresnel cosine integral function.
Proof
\(\ds \map {\operatorname C} x\) | \(=\) | \(\ds \sqrt {\frac 2 \pi} \int_0^x \cos u^2 \rd u\) | Definition of Fresnel Cosine Integral Function | |||||||||||
\(\ds \) | \(=\) | \(\ds \sqrt {\frac 2 \pi} \int_0^x \paren {\sum_{n \mathop = 0}^\infty \paren {-1}^n \frac {\paren {u^2}^{2 n} } {\paren {2 n}!} } \rd u\) | Power Series Expansion for Cosine Function | |||||||||||
\(\ds \) | \(=\) | \(\ds \sqrt {\frac 2 \pi} \sum_{n \mathop = 0}^\infty \frac {\paren {-1}^n} {\paren {2 n}!} \int_0^x u^{4 n} \rd u\) | Power Series is Termwise Integrable within Radius of Convergence | |||||||||||
\(\ds \) | \(=\) | \(\ds \sqrt {\frac 2 \pi} \sum_{n \mathop = 0}^\infty \paren {-1}^n \frac {x^{4 n + 1} } {\paren {4 n + 1} \paren {2 n}!}\) | Primitive of Power |
$\blacksquare$
Sources
- 1968: Murray R. Spiegel: Mathematical Handbook of Formulas and Tables ... (previous) ... (next): $\S 35$: Fresnel Cosine Integral $\ds \map {\operatorname C} x = \sqrt {\frac 2 \pi} \int_0^x \cos u^2 \rd u$: $35.21$