Offset logarithmic integral

The offset logarithmic integral is a non-elementary function Li(x) very similar to the logarithmic integral function li(x), defined such that:

Li(x) = li(x) - li(2).

Explicitly, this means

<math> {\rm Li} (x) = \int_{2}^{x} \frac{dt}{\ln (t)} \; . </math>

It is often used in formulations of the prime number theorem.