Notation
and mean that the sum or product is over all prime numbers:
Similarly, and mean that the sum or product is over all prime powers with strictly positive exponent (so 1 is not counted):
and mean that the sum or product is over all positive divisors of n, including 1 and n. E.g., if n = 12,
The notations can be combined: and mean that the sum or product is over all prime divisors of n. E.g., if n = 18,
and similarly and mean that the sum or product is over all prime powers dividing n. E.g., if n = 24,
Read more about this topic: Arithmetic Function