\begin{definition}[Definition:Cosine Integral Function]
The '''cosine integral function''' is the real function $\Ci: \R_{>0} \to \R$ defined as:
:$\map \Ci x = \ds \int_{t \mathop = x}^{t \mathop \to +\infty} \frac {\cos t} t \rd t$
\end{definition}