Divisor Sum of 840

Example of Divisor Sum of Integer

$\map {\sigma_1} {840} = 2880$

where $\sigma_1$ denotes the divisor sum function.

$\ds \map {\sigma_1} n = \prod_{1 \mathop \le i \mathop \le r} \frac {p_i^{k_i + 1} - 1} {p_i - 1}$

where $n = \ds \prod_{1 \mathop \le i \mathop \le r} p_i^{k_i}$ denotes the prime decomposition of $n$.

We have that:

$840 = 2^3 \times 3 \times 5 \times 7$

Hence:

$\ds \map {\sigma_1} {840}$

$=$

$\ds \frac {2^4 - 1} {2 - 1} \times \paren {3 + 1} \times \paren {5 + 1} \times \paren {7 + 1}$

$\ds $

$=$

$\ds \frac {15} 1 \times 4 \times 6 \times 8$

$\ds $

$=$

$\ds \paren {3 \times 5} \times 2^2 \times \paren {2 \times 3} \times 2^3$

$\ds $

$=$

$\ds 2^6 \times 3^2 \times 5$

$\ds $

$=$

$\ds 2880$

$\blacksquare$