The divisor function represented as $$d(n)$$ counts the number of a divisors of an integer

example: $$d(18)$$

The numbers that divide $$18$$ are $$1, 2, 3, 6, 9, 18$$ then $$d(18) = 6$$

## Important observations

• if $$p$$ is a prime number then $$d(p) = 2$$, also $$d(p^k) = k + 1$$ because every power of $$p$$ is a divisor of $$p^k$$, e.g. $$p^0, p^1, p^2, \ldots, p^k$$
• if $$n$$ is a product of two distinct primes, say $$n = pq$$ then $$d(pq) = d(p) \cdot d(q)$$, also $$d(p^iq^j) = d(p^i) \cdot d(q^j)$$
• in general let $$n = p_1^{a_1} \cdot p_2^{a_2} \cdot \ldots \cdot p_n^{a_n}$$ then $$d(n) = d(p_1^{a_1}) \cdot d(p_2^{a_2}) \cdot \ldots \cdot d(p_n^{a_n})$$ where $$p_i$$ is a prime factor that divides $$n$$

example: $$d(18)$$

\begin{align*} d(18) &= d(3^2 \cdot 2) \\ &= d(3^2) \cdot (2) \\ &= 3 \cdot 2 \\ &= 6 \end{align*}

int number_of_divisors(int n) {
int total = 1;
for (int i = 2; i * i <= n; i += 1) {
int power = 0;
while (n % i == 0) {
power += 1;
n /= i;
}
total *= (power + 1);
}
if (n > 1){
total *= 2;
}
}


## Sum of divisors

The sum of divisors is another important quantity represented by $$\sigma_k(n)$$, it's the sum of the $$k$$-th powers of the divisors of $$n$$

$\sigma_k(n) = \sum_{d|n} d^k$

examples:

\begin{align*} \sigma_0(18) &= 1^0 + 2^0 + 3^0 + 6^0 + 9^0 + 18^0 \\ &= 1 + 1 + 1 + 1 + 1 \\ &= 6 \end{align*}

So when $$k = 0$$ the sum of divisors ($$\sigma_0{n}$$) function is equal to $$d(n)$$, i.e. $$\sigma_0(n)$$ gives the number of divisors of $$n$$

another example:

\begin{align*} \sigma_1(18) &= 1^1 + 2^1 + 3^1 + 6^1 + 9^1 + 18^1 \\ &= 1 + 2 + 3 + 6 + 9 + 18 \\ &= 39 \end{align*}

when $$k = 1$$ we actually get the function we expect (a function which sums the divisors)

### Important observations

• if $$p$$ is a prime number then $$\sigma(p) = 1 + p$$ since the only divisors of a prime number are $$1$$ and $$p$$
• if $$p$$ is a prime number then $$\sigma(p^k) = 1 + p + p^2 + \ldots + p^k$$ because every power of $$p$$ is a divisor of $$p^k$$, e.g. $$p^0, p^1, p^2, \ldots, p^k$$

Consider

$\begin{equation}\label{sigma-p-k} \sigma(p^k) = 1 + p + p^2 + \ldots + p^k \end{equation}$

multiplying the expression by $$p$$ we have

$\begin{equation}\label{sigma-p-k-times-p} p \cdot \sigma(p^k) = p + p^2 + p^3 + \ldots + p^{k + 1} \end{equation}$

subtracting \eqref{sigma-p-k} from \eqref{sigma-p-k-times-p}

$p \cdot \sigma(p^k) - \sigma(p^k) = p^{k + 1} - 1$

factoring $$\sigma(p^k)$$

$\sigma(p^k) (p - 1) = p^{k + 1} - 1$

hence

$\sigma(p^k) = \frac{p^{k + 1} - 1}{p - 1}$

• if $$p$$ is a product of two distinct primes say $$n = pq$$ then $$\sigma(pq) = \sigma(p) \cdot \sigma(q)$$, also $$\sigma(p^iq^j) = \sigma(p^i) \cdot \sigma(q^j)$$

• in general let $$n = p_1^{a_1} \cdot p_2^{a_2} \cdot \ldots \cdot p_n^{a_n}$$ then $$\sigma(n) = \sigma(p_1^{a_1}) \cdot \sigma(p_2^{a_2}) \cdot \ldots \cdot \sigma(p_n^{a_n})$$ where $$p_i$$ is a prime factor that divides $$n$$

example: $$\sigma(18)$$

\begin{align*} \sigma(18) &= \sigma(3^2 \cdot 2) \\ &= \sigma(3^2) \cdot \sigma(2) \\ &= \frac{3^3 - 1}{3 - 1} \cdot \frac{2^2 - 1}{2 - 1} \\ &= 13 \cdot 3 \\ &= 39 \end{align*}

int sum_of_divisors(int n) {
int total = 1;
for (int i = 2; i * i <= n; i += 1) {
if (n % i == 0) {
int primePower = i;
while (n % i == 0) {
primePower *= i;
n /= i;
}
// sigma(n^k) = (n^{k + 1} - 1) / (k - 1)
total *= (primePower - 1) / (i  - 1);
}
}
if (n > 1) {
// if n is still a prime number after factorization
// sigma(n) = 1 + n
total *= (1 + n);
}