# Asymptotic Expansion for Fresnel Cosine Integral Function

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## Theorem

$\displaystyle \map {\operatorname C} x \sim \frac 1 2 + \frac 1 {\sqrt {2 \pi} } \paren {\map \sin {x^2} \paren {\frac 1 x - \frac {1 \times 3} {2^2 x^5} + \frac {1 \times 3 \times 5 \times 7} {2^4 x^9} - \ldots} - \map \cos {x^2} \paren {\frac 1 {2 x^3} - \frac {1 \times 3 \times 5} {2^3 x^7} + \ldots} }$

where:

$\operatorname S$ denotes the Fresnel cosine integral function
$\sim$ denotes asymptotic equivalence as $x \to \infty$.