Definition:Semiperfect Number/Mistake

Source Work

 * The Dictionary
 * $20$
 * $20$

Mistake

 * $20$ is the $2$nd semi-perfect number or pseudonymously pseudoperfect number, because it is the sum of some of its own factors: $20 = 10 + 5 + 4 + 1$.

It is in fact either the $3$rd or $4$th semiperfect number, after $6$ (if you count perfect numbers among the semiperfect numbers), $12$ and $18$.