Sum of Unitary Divisors of Power of Prime
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Theorem
Let $n = p^k$ be the power of a prime number $p$.
Then the sum of all positive unitary divisors of $n$ is $1 + n$.
Proof
Let $d \divides n$.
By Divisors of Power of Prime, $d = p^a$ for some positive integer $a \le k$.
We have $\dfrac n d = p^{k - a}$.
Suppose $d$ is a unitary divisor of $n$.
Then $d$ and $\dfrac n d$ are coprime.
If both $a, k - a \ne 0$, $p^a$ and $p^{k - a}$ have a common divisor: $p$.
Hence either $a = 0$ or $k - a = 0$.
This leads to $d = 1$ or $p^k$.
Hence the sum of all positive unitary divisors of $n$ is:
- $1 + p^k = 1 + n$
$\blacksquare$