I have an equation that contain exponential integral of the form:
$$ \begin{equation} E_k\left(\frac{a+b ~x}{c}\right) \end{equation} $$
Where $k\geq 0$ ($k=0,1,2,...$), $a$, $b$, and $c$ are constants, and $x>0$.
How to get the approximation of the above expression?