Divisor Sum of 28

Example of Sigma Function of Integer

 * $\sigma \left({28}\right) = 56$

where $\sigma$ denotes the $\sigma$ function.

Proof
From Sigma Function of Integer:
 * $\displaystyle \sigma \left({n}\right) = \prod_{1 \mathop \le i \mathop \le r} \frac {p_i^{k_i + 1} - 1} {p_i - 1}$

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

We have that:
 * $28 = 2^2 \times 7$

Hence: