Numbers whose Divisor Sum is Square/Examples/81

Example of Number whose Divisor Sum is Square

$\map {\sigma_1} {81} = 121 = 11^2$

Proof

From $\sigma_1$ of $81$:

$\map {\sigma_1} {81} = 121$

which equals $11^2$.

$\blacksquare$

Sources

• 1986: David Wells: Curious and Interesting Numbers ... (previous) ... (next): $81$
• 1997: David Wells: Curious and Interesting Numbers (2nd ed.) ... (previous) ... (next): $81$