Divisor function ($\tau (n)$)

The divisor function, often denoted as \( d(n) \) or \( \sigma_k(n) \), is a function in number theory that counts or sums the divisors of a positive integer \( n \). 1. **Count of Divisors**: The most common version is \( d(n) \), which counts the total number of positive divisors of \( n \).