This question
contains the equation $$ 2n-\sigma(n)=-12 $$ where $\ \sigma(n)\ $ is the sum of the divisors of $\ n\ $ including $\ n\ $.
There are two families of solutions :
- If $\ p>3\ $ is prime , then $\ 6p\ $ is a solution
- If $\ p=2^k-13\ $ , $\ k\ $ integer , is prime , then $\ 2^{k-1}\cdot p\ $ is a solution
The only solution upto $\ 10^9\ $ that fits in neither family, is $\ n=54\ $.
Is the list complete with the two families and the additional solution $\ 54\ $ ?