# Asymptotic Expansion for Cosine Integral Function

## Theorem

$\ds \map \Ci x \sim \frac {\cos x} x \sum_{n \mathop = 0}^\infty \paren {-1}^n \frac {\paren {2 n + 1}!} {x^{2 n + 1} } - \frac {\sin x} x \sum_{n \mathop = 0}^\infty \paren {-1}^n \frac {\paren {2 n}!} {x^{2 n} }$

where:

$\Ci$ denotes the cosine integral function
$\sim$ denotes asymptotic equivalence as $x \to \infty$.