Logarithmic integral
From specialfunctionswiki
The logarithmic integral is $$\mathrm{li}(x) = \displaystyle\int_0^x \dfrac{dt}{\log(t)},$$ where $\log$ denotes the logarithm. The logarithmic integral is related to the exponential integral by the formula $$\mathrm{li}(x)=\mathrm{Ei}( \log(x)).$$
