Prime-Counting Function in terms of Eulerian Logarithmic Integral

Theorem
Let $\map \pi x$ denote the prime-counting function of a number $x$.

Let $\map \Li x$ denote the Eulerian logarithmic integral of $x$:
 * $\map \Li x := \displaystyle \int_2^x \dfrac {\d t} {\ln t}$

Then:
 * $\map \pi x = \map \Li x + \map {\mathcal O} {x \, \map \exp {-c \sqrt {\ln x} } }$

where:
 * $\mathcal O$ is the big-O notation
 * $c$ is some constant.