Logarithmic integral
From specialfunctionswiki
The logarithmic integral is $$\mathrm{li}(x) = \displaystyle\int_0^x \dfrac{dt}{\log(t)},$$ where $\log$ denotes the logarithm. It can be shown that $\mathrm{li}(x)=\mathrm{Ei}( \log(x)),$ where $\mathrm{Ei}$ denotes the exponential integral.
