Editing Trigonometric integral (section)

== Relation with the exponential integral of imaginary argument ==
The function
<math display="block">\operatorname{E}_1(z) = \int_1^\infty \frac{\exp(-zt)}{t}\,dt \qquad~\text{ for }~ \Re(z) \ge 0 </math>
is called the [[exponential integral]]. It is closely related to {{math|Si}} and {{math|Ci}},
<math display="block">
\operatorname{E}_1(i x) = i\left(-\frac{\pi}{2} + \operatorname{Si}(x)\right)-\operatorname{Ci}(x) = i \operatorname{si}(x) - \operatorname{ci}(x) \qquad ~\text{ for }~ x > 0 ~.
</math>

As each respective function is analytic except for the cut at negative values of the argument, the area of validity of the relation should be extended to (Outside this range, additional terms which are integer factors of {{math|''π''}} appear in the expression.)

Cases of imaginary argument of the generalized integro-exponential function are
<math display="block">
\int_1^\infty \cos(ax)\frac{\ln x}{x} \, dx =
-\frac{\pi^2}{24}+\gamma\left(\frac{\gamma}{2}+\ln a\right)+\frac{\ln^2a}{2}
+\sum_{n\ge 1} \frac{(-a^2)^n}{(2n)!(2n)^2} ~,
</math>
which is the real part of
<math display="block">
\int_1^\infty e^{iax}\frac{\ln x}{x}\,dx = 
-\frac{\pi^2}{24} + \gamma\left(\frac{\gamma}{2}+\ln a\right)+\frac{\ln^2 a}{2} 
-\frac{\pi}{2}i\left(\gamma+\ln a\right) + \sum_{n\ge 1}\frac{(ia)^n}{n!n^2}  ~.
</math>

Similarly
<math display="block">
\int_1^\infty e^{iax}\frac{\ln x}{x^2}\,dx = 
 1 + ia\left[ -\frac{\pi^2}{24} + \gamma \left( \frac{\gamma}{2} + \ln a - 1 \right) + \frac{\ln^2 a}{2} - \ln a + 1 \right]
 + \frac{\pi a}{2} \Bigl( \gamma+\ln a - 1 \Bigr) 
 + \sum_{n\ge 1}\frac{(ia)^{n+1}}{(n+1)!n^2}~.
</math>