# Power Series Expansion for Fresnel Cosine Integral Function

## 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

$\blacksquare$