Erdős-Borwein Constant
From specialfunctionswiki
$$E = \displaystyle\sum_{k=0}^{\infty} \dfrac{1}{2^k-1} = 1.606695152415291763\ldots$$ Note that the numbers in the denominator in this sum are the Mersenne numbers.
Properties
Erdős-Borwein Constant is irrational