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?