# §6.6 Power Series

 6.6.1 $\mathop{\mathrm{Ei}\/}\nolimits\!\left(x\right)=\gamma+\mathop{\ln\/}\nolimits x% +\sum_{n=1}^{\infty}\frac{x^{n}}{n!\thinspace n},$ $x>0$.
 6.6.2 $\mathop{E_{1}\/}\nolimits\!\left(z\right)=-\gamma-\mathop{\ln\/}\nolimits z-% \sum_{n=1}^{\infty}\frac{(-1)^{n}z^{n}}{n!\thinspace n}.$
 6.6.3 $\mathop{E_{1}\/}\nolimits\!\left(z\right)=-\mathop{\ln\/}\nolimits z+e^{-z}% \sum_{n=0}^{\infty}\frac{z^{n}}{n!}\mathop{\psi\/}\nolimits\!\left(n+1\right),$

where $\mathop{\psi\/}\nolimits$ denotes the logarithmic derivative of the gamma function (§5.2(i)).

 6.6.4 $\mathop{\mathrm{Ein}\/}\nolimits\!\left(z\right)=\sum_{n=1}^{\infty}\frac{(-1)% ^{n-1}z^{n}}{n!\thinspace n},$
 6.6.5 $\mathop{\mathrm{Si}\/}\nolimits\!\left(z\right)=\sum_{n=0}^{\infty}\frac{(-1)^% {n}z^{2n+1}}{(2n+1)!(2n+1)},$ Symbols: $!$: factorial (as in $n!$), $\mathop{\mathrm{Si}\/}\nolimits\!\left(\NVar{z}\right)$: sine integral, $z$: complex variable and $n$: nonnegative integer A&S Ref: 5.2.14 Referenced by: §6.5 Permalink: http://dlmf.nist.gov/6.6.E5 Encodings: TeX, pMML, png See also: Annotations for 6.6
 6.6.6 $\mathop{\mathrm{Ci}\/}\nolimits\!\left(z\right)=\gamma+\mathop{\ln\/}\nolimits z% +\sum_{n=1}^{\infty}\frac{(-1)^{n}z^{2n}}{(2n)!(2n)}.$

The series in this section converge for all finite values of $x$ and $|z|$.