Editing Exponential integral (section)

===Generalization===
The exponential integral may also be generalized to

:<math>E_n(x) = \int_1^\infty \frac{e^{-xt}}{t^n}\, dt,</math>

which can be written as a special case of the upper [[incomplete gamma function]]:<ref>Abramowitz and Stegun, p.&nbsp;230, 5.1.45</ref>

: <math>E_n(x) =x^{n-1}\Gamma(1-n,x).</math>

The generalized form is sometimes called the Misra function<ref>After Misra (1940), p.&nbsp;178</ref> <math>\varphi_m(x)</math>, defined as

:<math>\varphi_m(x)=E_{-m}(x).</math>

Many properties of this generalized form can be found in the [https://dlmf.nist.gov/8.19 NIST Digital Library of Mathematical Functions.]

Including a logarithm defines the generalized integro-exponential function<ref>Milgram (1985)</ref>
:<math>E_s^j(z)= \frac{1}{\Gamma(j+1)}\int_1^\infty \left(\log t\right)^j \frac{e^{-zt}}{t^s}\,dt.</math>