Definition:Semiperfect Number/Mistake

Source Work

1986: David Wells: Curious and Interesting Numbers:

The Dictionary

$20$

1997: David Wells: Curious and Interesting Numbers (2nd ed.):

The Dictionary

$20$

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 $18$.

Sources

• 1986: David Wells: Curious and Interesting Numbers ... (previous) ... (next): $20$
• 1997: David Wells: Curious and Interesting Numbers (2nd ed.) ... (previous) ... (next): $20$