Why is pi used to represent prime numbers here? I've just begun the "Concrete Mathematics" book by Knuth et al. In the first section about sums (and I apologise if this is really trivial, but I'm new and struggling a little);
and they show this as one of the examples:
$\sum_{k = 1}^{\pi(N)}  \frac{1}{p_{k}}$
"This is the sum of all reciprocals of prime numbers between 1 and N."
If you didn't have the explanation here, how would you know that N is supposed to be a prime number? Is it because of the pi symbol? Also, does pi often represent a number as prime?

$N$ is not supposed to be a prime number. $\pi(N)$ denotes the number of primes $\leq N$ and is (obviously) a function of $N$. Similarly, $p_k$ is the $k$th prime . 
