How to count multiplicity the number of combinations of divisors of $n$ if the number of proper divisors of $n$ is $2p-1$? Note that $n$ is a perfect number.
I did tried combinations, since the number of elements is $2p-1$, so the number of combinations will be using the sum of divisors from combinations.
Since we need to exclude empty combinations, we subtracted $1$. But I got so confused about the reason why it was done that way.