# Definition:Fresnel Cosine Integral Function

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## Definition

The Fresnel cosine integral function is the real function $\R \to \R$ defined by:

$\ds \map {\operatorname C} x = \sqrt {\frac 2 \pi} \int_0^x \cos u^2 \rd u$

### Asymptotic Expansion for Fresnel Cosine Integral Function

 $\ds \map {\operatorname C} x$ $\sim$ $\ds \frac 1 2 + \frac 1 {\sqrt {2 \pi} } \paren {\map \sin {x^2} \paren {\sum_{n \mathop = 0}^\infty \paren {-1}^n \frac {\paren {2 n + 1}!} {2^{3 n} n! x^{4 n + 1} } } - \frac 1 2 \map \cos {x^2} \paren {\sum_{n \mathop = 0}^\infty \paren {-1}^n \frac {\paren {2 n + 1}!} {2^{3 n} n! x^{4 n + 3} } } }$ $\ds$ $=$ $\ds \frac 1 2 + \frac 1 {\sqrt {2 \pi} } \paren {\map \sin {x^2} \paren {\frac 1 x - \frac {1 \times 3} {2^2 x^5} + \frac {1 \times 3 \times 5 \times 7} {2^4 x^9} - \ldots} - \map \cos {x^2} \paren {\frac 1 {2 x^3} - \frac {1 \times 3 \times 5} {2^3 x^7} + \ldots} }$

## Also see

• Results about the Fresnel cosine integral function can be found here.

## Source of Name

This entry was named for Augustin-Jean Fresnel.