Ramanujan–Soldner constant

The Ramanujan - Soldner constant is a mathematical constant, which is defined as the only positive zero of the logarithmic integral li. It is named after S. Ramanujan and Johann Georg von Soldner, it had been investigated already in 1792 by Lorenzo Mascheroni.

An approximate value is

There are 75,500 decimal digits known (as of August 2010).