Editing Exponential integral (section)

===Derivatives===

The derivatives of the generalised functions <math>E_n</math> can be calculated by means of the formula <ref>Abramowitz and Stegun, p.&nbsp;230, 5.1.26</ref>
:<math>
E_n '(z) = - E_{n-1}(z)
\qquad (n=1,2,3,\ldots)
</math>
Note that the function <math>E_0</math> is easy to evaluate (making this recursion useful), since it is just <math>e^{-z}/z</math>.<ref>Abramowitz and Stegun, p.&nbsp;229, 5.1.24</ref>