TITLE: What is $B'_L$ and why is it equal to 1?
QUESTION [7 upvotes]: I understand all the other ones, but the $B'_L$ has me stumped. What does it mean and why is it equal to 1?

REPLY [9 votes]: It seems this is Legendre's constant
$$
  B^\prime_L = \lim_{n \to \infty}\left ( \log n - \frac{n}{\pi(n)} \right)
$$ 
where $\pi(n)$ stands the number of primes not exceeding $n$.